All good Kevin, I placed my order tonight for a VTF-15H MK2. I suspected as much, in that the TNs should be a bottleneck... Not that I'm hurting in output or extension as you can see from the graphs.
I've attached some Room EQ Wizard plots after EQ, and before EQ showing that subwoofers placed 10' apart can still have great output summation. This is my 2 TN1220HO subs where I put them 16 years ago; one in each back corner of the room. Every room is different and all that, but if you've got dumb-luck like I did, you can see I was getting pretty good real-world in-room response/summation to start with. Yeah, there's some nasty nulls there because the real world sometimes interferes with our plans.
Red line is back left sub, green line is back right sub, blue is both at same time. The subs are gain matched, but I have no delay added as it makes overall response worse by some miracle of placement relative to listening position. I then saw the "natural house curve look" to it, and set a +10dB desired house curve in REW and it generated EQ filters that did what they said they'd do to give the REV 3 EQ graph after I imported the filters into my MiniDSP 2x4HD.
I did gain matching at -25dB, with both subs placed in the same spot in the middle of the room, and adjusted to 85dB. I must have had them placed in a null, or the tones generated by my receiver were at about 48Hz...
I may have to use REW to see what the frequency of the subwoofer test tones from my receiver are at frequency wise now that I'm thinking about it.
I'm probably going to get the tape measure out and get the exact dimensions of the room and do some simulations to see where 3 subs gets the flattest response and try positioning subs there. I suspect it'll end up something like front left, back right, and somewhere on a side wall as best options. With only 1 big heavy sub to move (the TN's are featherweights and easy to move), it's not too bad. If I didn't have a fireplace in the front right corner, I'd likely have just done a sub in each corner much to my wallet's dismay.
Anyway, just wanted to say thanks for the advice, and wanted to show proof that even the older HSU subs are still quite potent. Oh yeah, I still want updated TN subs.