Why, one front/one back sub, a double distance?

I have two subwoofers, both in an infinite baffle configuration (mounted in the wall, into the outside world or attic space, with,yes, a response flat below 10 htz:;and there is no other way to get this at any price).

One is in the front middle of the room (two long-throw 15"), the other in the back, right hand corner (one 18" very long throw Ficaraudio, and two very capable long-throw 10").

My Onkyo pre/pro calculates the 'distance' of the 'subwoofer' as 24 feet, exactly half of the distance from the listening position from each.

The bass is MUCH better if I override the Onkyo calculation by dividing it in half, the actual distance between the two.

What's the explanation?
No double distance. If you sit equidistant from the two (one sub in front and one behind) then you should enter that distance (from you to one of the subs) in your pre/pro. Also make sure the subs are in phase. This will ensure that sound from your sub arrives in phase with that from your main speakers (assuming you put the correct distance into your mains on the pre/pro)

Finally without performing actual measurements with a microphone and a sweep I don't think you can even begin to know what the actual sound is like at the listening position. If it sounds better by putting in the wrong distance then you have room modal issues or placement issues of a sub is wired out of phase.

Typically, Audyssey will measure the "distance" of the subwoofer(s) as significantly longer than the physical distance because the delay, not the distance, is the critical measure. In the real world, that delay is sum of the physical distance and the time-delay introduced in the processing circuitry of the sub channel.

Hi Pmcneil, you said "a response flat below 10 htz:;and there is no other way to get this at any price)."

That is a great lowwww bass response, how did you measure this? And I would love to see a graph of it. My sub is flat to 12hz, I use the free program from the Home Theater Shack "REW" with a calibrated mic. What program do you use?

Have you tried running the rear sub "anti-phase"?