I have made progress with the integration of the fourth ULS-15. In a previous post, I described how I am having trouble with the trims that Audyssey has been setting for the subs. The two front subs have a trim of +7.5, and the two rear, co-located subs have a trim of -3.5. The 11dB difference has been bothering me, and the overall quality of the bass seems lacking, even though the frequency response is acceptably flat.
The more I thought about it, I began questioning the gain-matching approach I have been using. I decided to try a different approach: treat the two co-located rear subs as "one virtual sub". Compare the two ULS-15's with a single Seaton Submersive, which has two opposing sealed 15-inch drivers in one cabinet. If I were gain-matching the Submersive to the two front ULS-15's, the current gain on the front subs would actually be 6db too low. Co-locating the two ULS-15's raises the output level by approximately 6dB, so if you were to consider this a one "virtual sub", then it is really no longer gain-matched with the two front subs. To re-establish the gain-matching, I raised the gains on each of the two front subs by 6dB so that their output more closely matches the rear "virtual sub".
Now, when I run the Audyssey calibration, the front trims are set to -.5dB, and the rear "virtual sub" trim is set to -3dB. Since the rear subs closer at only 3.5 feet from the MLP, compared with the front subs at 11 feet, this result is much more consistent with what I had been expecting. The frequency response graph is also virtually unchanged.
I haven't thought through what this different approach means to the combined headroom of the four subs, but I do hear a difference in how the bass sounds, and the difference is very positive. I would describe the current sound as more enveloping, smoother, and stronger. Now I need to run through my go-to demo material to confirm this initial impression, but my feelings are very positive.