What process did you use to integrate multiple subwoofers for 2 channel listening?

The problem I’m not able to solve, and perhaps the room’s physics make it impossible, is a +5db or more bass hump from about 73-92 hz. This is evident in REW and it’s also easily noticed with music. A tubby zone. The idea that one more sub would solve this is implausible. It will take a systematic approach to moving the subs and adjusting various things, and I'd bet a few hundred $$s that that fails too. But at least I'd know I had hit the limits of this room. Thanks for any approaches. I’ll ask over at the REW forum, too.

I think you've nailed it your thread description - trial and error. It takes a lot of time to go through the various iterations, but viewing the measurements is well worth the effort.
If you haven't done it already, go to AVS forums and to the Rythmik thread and download Enrico's tip for dialing in their subs (it's in his signature).
Don't be afraid to use different volume adjustments, different frequencies, different bass extension filters for each sub. Experiment with room locations (ie different angles, even pointing the front of the sub to the wall).
Take notes (I added them to REW). It's all important so you know what works and what doesn't. Taking notes will help speed things up. Carpet slides are a great idea.

Are you running any software you can apply EQ, should be fairly easy to notch
 70-90hz.  If you have the PEQ version of the Rythmiks put a -6db at 80hz,  measure and adjust. 

The problem I’m not able to solve, and perhaps the room’s physics make it impossible, is a +5db or more bass hump from about 73-92 hz.

Well, this is easily solved by an EQ, and that is a relatively small hump, I've measured +20 in a modest living room at ~ 25 and 50 Hz. 

This hump is also in the range of bass traps such as GIK's.  Clipping it will enable you to bring the bass level up and sound more full and balanced.

Rather than try to solve this with multiple subs I would strongly encourage a mix of bass traps and EQ.

I’m using a pair of 15" Rythmiks to supplement bass on my Avantgarde Duos, which have integrated woofers that do not go down extremely deep- when they are dialed up for more bass impact, they sound discontinuous with the mid-horn; when adjusted to blend with the midhorn, they sound anemic. The Rhythmic subs address that pretty effectively.
My task was made easier by focusing on the blend between the integrated woofers and the subs-- partly adjusted by positioning of the woofers (I had help in the form of two college football player types who did the heavy lifting). I also used a cheap little DSP unit- the Dspeaker 8033-- which is surprisingly effective to even out the bass in a decent sized room.
Once I ran the cycles on the DSP unit and it lopped off some peaks, I then zero’d in more-- lowered crossover frequency, adjusted slope and played with phase to get them to cohere. I had help here-- my wife- who adjusted gain, crossover and phase with direction from me sitting in normal listening position.
The system is very resolving at low volume and when the subs energize the room, you know it! But, you almost would not know they were there unless I shut them off. (FWIW, they run from a separate line out on my line stage, so the DSP is not in the same circuit as the main speakers). Even if you do measurements, final tuning is probably going to be by ear, no? Once set, I’m done. I do not mess with the settings for different kinds of music. The DSP unit sums the channels so it's mono. I know there's an argument there, but it is way easier to do and my main source is LP, which is often summed below a certain frequency anyway.