Stereo Bass Management & A Distortion Dilemma: Toward An Acceptable Compromise (Update: [SOLVED])

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 Update: Solved*

A few nights ago, I turned out the lights and turned up the volume on my Onkyo TX-SR706 to listen to Landscape In Portrait by Marihiko Hana. As you can tell from the video below the style is mainly solo piano, and a fair minimal one at that:

Very soon after starting up the album, I was greeted by some rather non-musical distortion emanating from my speakers. It sounded like brief tearing noises that were produced when a piano note was played loudly with some attack. Initially I thought it may just be an issue with the recording, perhaps it was clipped and was meant to sound that way. However, the distortion kept returning and was terribly non-musical. I switched over to listen via my pair of Audio Technica ATH-M50x headhones and the distortion disappeared.

That's odd, I thought, what was going on here. Were the headphones not revealing enough? Are my speakers blown? Is there an issue with the amp? Needless to say I was worried and that some investigation was surely in order.

Googling "distortion" and "piano music" I came across some forum threads where users argued that piano music is notoriously difficult to capture and reproduce. The instrument is capable of tremendous dynamic range and can easily overload an improperly configured microphone. The dynamic range can also cause issues with amplifiers that are not up to the task of delivering clean wattage for the highest peaks. To me, the recording seemed fine (no issues in the waveform that I could detect) and I was still hearing the distortion at low volumes. This suggested to me that it wasn't a problem with driving the amp or speakers too hard.

Still, I was concerned. I was able to quickly test the same files on a wholly different receiver/speaker setup and lo and behold: distortion! Now I could be reasonably sure my speakers aren't damaged (phew). Was the amp the culprit? Why did it sound fine in my headphones?

A little more deep googling alerted me to another thread where an audio enthusiast was having trouble with his signal processor.  Their distortion issues went away after replacing it. I have no other means of using another processor with my A/V receiver. Then I got the idea to try the "Pure Audio" mode, which passes the signal from the input stage to the amplifiers with as little processing as possible.

It worked. The music was crystal clear with no audible distortion. Everything sounded as it should. Except... no subwoofer.

The Pure Audio mode (and Direct mode on this particular receiver) disables the bass management filters and processors. I am quite fond of listening to stereo music in a 2.1 configuration with a subwoofer appropriately crossed over with the speakers (I actually use a 5.1 setup, though I rarely choose to upmix stereo to surround using built-in Dolby or DTS algorithms). I like my subwoofer and I like hearing (er, feeling) the lowest frequencies of the program material. Engaging the receiver's crossover filter immediately introduced the awful distortion, and bypassing it instantly solved the issue. But what about the subwoofer?

Fortunately, this receiver has a Double Bass feature. This means in Stereo mode, which is usually used to engage crossover and other settings, I can run my front left and right speakers in Full Range mode and have to receiver also send bass information to the subwoofer via the LFE output. Instead of some bass being routed to the sub away from the main channels, the speakers get the full bandwidth signal and the sub gets some love too. And there is no distortion!

I have no idea why this distortion is introduced in Stereo mode with bass management engaged. It does not distort with bass management engaged when I'm using Dolby Pro Logic II or All Channel Stereo modes. This is bizarre.

Whatever the cause, I was on my way to a solution. I had been using the receiver's bass management system to control information routed to the subwoofer and the settings I used performed really well. Using Room EQ Wizard, I performed low frequency sweep tests to measure the response from the listening position. Here's what I'd be using all along:

Bass management on receiver, crossed over at 80Hz

Using the built in microphone on a laptop (I know, not an ideal measurement tool), I can get a good idea of how the system is performing. These graph is useful for confirming what I'm hearing when I do the bass tests. It's fairly smooth in the prime listening position, which is more than can be said for elsewhere in the room...

But if I want to avoid the distortion caused by the stereo bass management in the receiver, I must deviate from this setup. First, here are some other measurements to provide context for performance and settings:

Running the two main front speakers full range; no subwoofer or bass management

Bass management on receiver, crossed over at 50Hz

As you can see, running my SVS Ultra Towers at full range leaves a little to be desired when it comes to achieving flat response in my somewhat large room. The speakers are rated at 28 Hz-32 kHz (+/-3 dB), so they are certainly capable of delivering serious low end response. However, the laws of science be a harsh mistress. The SVS PB-1000 subwoofer helps pick up the slack when it comes to exciting the room in the lowest frequency ranges. Crossed over at 50Hz, the sub helps bring up the levels and results in a flatter curve; but the 80Hz point is really tough to beat, though some may deem it a little high for use with these particular speakers.

Now it is time to dial in the Double Bass settings to see if I can get a roughly comparable curve. The subwoofer has it's own dial for setting the crossover point. The filter can be set anywhere between 50-160Hz. For the first test, I ran the front stereo speakers at full bandwidth with Double Bass engaged and the filter on the subwoofer set to it's maximum height (160Hz, or "disable/LFE").

Stereo speakers full bandwidth, double bass active, highest crossover setting on subwoofer (160Hz/disable)

It's a start, but that's a nasty trough at 60Hz. There's a slight peak beyond that, but nothing to be overly concerned about. Let's keep trying...

Stereo speakers full bandwidth, double bass active, lowest crossover setting on subwoofer (50Hz)

Turning the crossover dial all the way down to 50Hz yields a nice curve. There's still a dip between 50-60Hz, but it's not nearly as dramatic as the previous setting. I think we can do better than this. After all, we already have a good idea of what the appropriate setting might be given the positions of the speakers/subwoofers haven't changed, nor have I adjusted any gain levels.

Stereo speakers full bandwidth, double bass active, crossover at ~80Hz on subwoofer

Now this is much better. It ain't perfect, nor quite as smooth as the original setting with the receiver's bass management enabled, but it's fairly close. We still have the slight dip and peak in response in the 50-80Hz range, which could probably be targeted if I were so inclined to use some precision equalization. But let's not get carried away here.

Just for fun, Room EQ wizard can overlay all the response measurements for comparison:

The two outliers are the Full Range with no subwoofer active and the Double Bass with the highest crossover setting on the subwoofer curves. The other graphs are actually fairly similar, especially beyond the 60Hz point. Judging by these measurements, it seems that enabling Double Bass and running the speakers full range with the subwoofer's crossover setting at about 80Hz is a perfectly acceptable compromise. It's not ideal; I'd much rather have a true crossover between the main channels and the subwoofer, and hopefully this configuration is only temporary. I certainly do not want to hear that distortion ever again, and I certainly do want to continue using my subwoofer with stereo music. I mean, have you heard the new Dirty Projectors record!?

Update 

Upon further consideration and some general listening, the Double Bass option is untenable. While the graphs look good as far as room response is concerned, the sound is just too muddy. I guess I'll just run everything in Direct mode until I find another solution, or go back to the original bass management settings and switch it off if I begin to notice distortion. The full bandwidth option with no bass management is fine for piano music, and I've only noticed the distortion once or twice before on albums with full band arrangements (though I originally thought the recording was at fault, I have since confirmed Direct mode clears up the trouble spots I'd heard elsewhere prior to this glaring issue).

Update The Second

Solved.