Jim, are you agreeing with my sub placement as an acceptable trade off.
Unfortunately, I can't agree or disagree. Without being in your room, and seeing what placement works best, I can't possibly know.
Have you tried inverting the polarity on all 4 subs, to see if that's an improvement? Sometimes with them located behind the horns, it'll help.
If you can't run pink noise with a 1/3 octave RTA, then play some recordings with simply miked solo vocals. If female vocal, use one that's fairly low. The ubiquitous Diana Krall can work.
Listen with all of the subs in one polarity. Note how present the vocalist is.
Then invert the subs. Without adjusting the volume control, play the same recording again.
Very often, one way will sound a bit hollow, and the other way will sound more palpable and present. Hollow is phase cancellation, so you don't want that configuration.
If you're using a jumper to go over to a pair of subs per channel, just invert the polarity at the amp end. It'll take care of both. Obviously, do this for both channels.
How do you set your subs and horns in your room. Could you please include dimensions.
What happens in any room is only a function of that room, and usually will have little bearing on another installation. So I'd prefer not to introduce any potentially confusing tips that may likely cause more trouble than they solve...
Incidentally, this technique is not at all unique to Avantgardes. I remember going through a similar process with the Mark Levinson HQD (Hartley subs, Quad mids, Decca tweeters) system in 1978. And countless systems since.
A lot has changed since then, but the basic laws of physics still have to be obeyed... :)
Best regards,
Jim Smith