Originally Posted by Stereodude
The base of a pair of tower speakers is not likely to be the best place acoustically in a room for a pair of subwoofers. As such, DIY creations aren't likely to put a subwoofer in a tower speaker.
Generally true if you're only using one or two subs. But people interested in high fidelity bass reproduction generally use more than two subs. (Some rooms can do just fine with two subs blended into the mains.)
Wayne Parham (Pi Speakers guy) has some useful ideas about about using flanking subs
as part of a multisub system. The main benefit is that the flanking subs can play higher, so you can work on floor bounce issues by blending the flanking subs and mains, a la Allison. (The NHT 3.3 did something similar, albeit passively.)
But the best way to do flanking subs, IMO, is not to make big tower cabinets with built-in subs, but to make "standmount" speakers and have the flanking subs in separate identical-looking cabinets, which can be used as stands. The advantages are that they're easier to move, one can still have three identical front mains with just two flanking subs, one can limit vibration to the top cabinet by using Sorbothane spacers, and one can fine-tune the in-room response by experimenting with sub firing angle.
My currently-in-the-works mains are following that route. There will be two or three additional subs in the room, depending on what's needed to smooth out response.