[Wayne A. Pflughaupt]

Welcome to the Forum!

This might be better suited for our Acoustics Forum, but we'll see where the discussion takes us before deciding whether or not to move it there.

We have a dozen more graphs showing different sub positions, speaker positions, sound absorption configurations, etc. without getting anything better than this.

I would think that the listening position is the only one to be concerned about. What does it matter if a foot or so over the response is bad if that's not where you're listening from?

The 2nd graph shows the problem we are having smoothing the peaks and valleys at even the normal listening position. We thought that the valleys were nulls and the peaks resonances.
But the 2nd graph shows that a 4 dB increase in sound level raised all portions of the spectrum about equally.

That tells me that what you're seeing there aren't nulls and therefore should be equalizable.

We would prefer treat the room...

Well, the folks who sell bass traps will tell you that if you put enough of them in a room, it'll pretty much eliminate depressions in response (or at least greatly reduce them). Of course the problem there is that you'll have no more room in the room...

...and not use a PEQ, because the room is used for recording as well as playback, and the PEQ won't help with the recording.

Not following you there. Why would PEQ to optimize response at the listening position have any effect on recording? Typically electronic instruments are direct-input. Few acoustic instrument are going to get down to where the 40 Hz problem is.

Regards,
Wayne