Here's another way to look at it. Your room contributes a lot of the output at the lowest frequencies, so in terms of SPL capability, proximity doesn't matter all that much for ULF. Indeed, near-field placement may give inferior ULF performance vs. a rigid corner, for example. For mid bass frequencies, the room is less important and proximity matters. Mid bass can be a lot louder and cleaner with near-field placement. So in general, if you want more/better ULF, then you probably want to match the displacement capability that you already have. You may want to experiment, possibly by trialing one or both 24 close by to see how good the ULF is with the sub next to your seat vs. up front. If you want more mid-bass and punch, then you can get away with less displacement capability. Instead, go for drivers that are light and low Le but still big, like pro drivers. Build ported boxes to push the efficiency higher still. The right drivers will be able to hit harder with a lot less power than 24s up front require. IMO, near-field is better for mid-bass but not ULF. Other opinions vary.