I've written lots about this.
I have 5 in the main room and 2 in the living room. Two more 500s in walnut espresso and boston cherry are coming. All v4.
The main room has a black oak 800 front and centre, two 600s in walnut caramel on the side and two 500s in walnut natural in the back. One 800 would have been enough for anyone. I bought the two 500s with my active LFRs and immediately noticed more spatial diversity with the bass. With the current topology and the 600s added to the mix, there's a holy level of saturation simply not possible with any single sub. Of course there's also the headroom but that was never an issue even with just a single 800.
I have two 125s in the living room flanking opposing corners for use with the M5OW. With those, the bass is more mid-bassy (50 to 80 Hz) given the 125 response curve. What's very interesting however is just how textured the bass is even though the mid-bass just pops! I know it's not right but I don't care. It's totally addictive in its own way. So I have smooth, deep, saturating bass downstairs and pounding mid-bass upstairs. But both are very articulate.
The two new 500s will go in the living room and I'll see how they behave with the 125s. Andrew has said multiple subs should all be of "equal weight" but I think if you're careful in placement and adjustment, you can make it work very well. So maybe I can get both mid-bass pound and depth. We'll see.
BTW, I can hear growls and other effects from string bass with the sealed subs which I can't hear from SVS, Hsu and Klipsch/Polk/KEF (not even in the same league as the others). The 125 as well but they can't get as low and hit harder in the mid-bass.
I just became aware mid last year that Axiom made subs, of uniquely different design, that it hasn't announced. Likely they were custom. I think there's a lot of skunkworks going on over there that we're simply not privy to. I can't substantiate it but rumours have it that Ian has a lab in the Kamchatka Penninsula where he caters to the Soviet elite. I've heard stories of other labs too but it's all just speculation and conjecture.
