Just finished getting caught up on this thread!
I think you have evolved past this point, but just in case...Would one of your Hsu's fit better in the near-field location? My first thought was to put the RBH in the location you said worked best for a sub in your room. Just another consideration that you have probably already thought of, but I know sometimes when I get wrapped up in getting something done, I don't always take that step back to consider other options. However, it does make sense to keep your Hsu's up front to support Audyssey in tuning your system (see below).
Here is my understanding on bi-amping (please anyone correct me if I am mistaken).
Why standard bi-amping is of little benefit
Usually with a 2 or three way speaker, if you look at the phase-impedance curve there is one (sometimes 2) place where a speaker is a glutton for power - let's say at 45Hz where the impedance drops down to 3 ohms and the phase angle is at 40 degrees. This is where that 100Watt AVR is likely to find its limit. If you have extra channels on your AVR and if the speaker allows it, you can bi-amp the speakers, but that is little to no help with the problem of 100 Watts into the frequency where impedance and phase angle are stacked against you. You can make a case that the tweeter is not going to sound as bad since the amp driving it is not the one being clipped, but the woofer amp is still being pushed into clipping. So, you are better off buying one 200 (or 400!) Watt amp than buying two 100Watt amps and bi-amping.
In your case, you are driving identical drivers (with identical phase-impedance charts), so I think there is more benefit to bi-amping (bi-amping has marginal benefit with a standard speaker, just not much and if you have options, getting a single more powerful channel is a better solution). I would expect the Monolith provides enough power for typical listening and bi-amping will certainly provide a little more headroom.
Since you have
Audyssey XT32, you should make your 2 Hsu subs one channel (using a RCA splitter) and your RBH the other channel (again using a RCA splitter if you bi-amp). The only place where Audyssey differentiates between the channels is matching the level and delay. With the Hsu's approximately the same distance from the LP (?) you can manually match volumes (if the gain controls and amps are consistent, or you can do it by ear if they are not) and they should be in phase with each other (again assuming roughly same distance to the LP). This is the SubEQ feature of Audyssey XT32 (some earlier Onkyo amps had Audyssey XT32 without subEQ which forced you to manually level/delay match two subs just as I suggest here). Obviously (if you bi-amp) the 2 channels of the RBH should be well matched for level and phase and if you don't bi-amp, it is a mute point.
Next, the subEQ will match level and phase between the HSU's and the RBH. Then Audyssey treats all 3 (or "4" if bi-amped) subs as one when it is time for the EQ room correction.
I don't know how your MiniDSP tuning worked best, but Audyssey concluded it was better to treat all subs as one than to attempt to tune them separately!
You probably already know all of this, but it has been a while since I have posted and I was feeling left out of the fun!!!
