Ian,
All the forgoing recommendations are good ones.
VMPS, HSU, SVS, and Sunfire.
To that list, I would add the Earthquake SuperNova MKII 15". I have one and have had all the above but the SVS in my system and must say that it equals or surpasses all of them for Home Theater.
http://www.earthquakesound.com/serv02.htm
If you are looking for the best hybrid (music and theater) then it narrows the field.
VMPS and Earthquake are the clear leaders (IMHO) in my listening enviroment.
The VMPS is $699 (no amp) the SuperNova is $2500 with a 580 watt amp.
Stereophile will have a review coming out soon on the SuperNova and wait till you see what they have to say. It has also been well reviewed in other publictions for its power and SPL. (sound pressure levels) as well as accuracy and quickness.
The VMPS is called the "LARGER" and it is (a 12" and 15" driver with a 15" passive radiator). If you couple this with a well dampened 300+ watt amp you will hit 16hz with more authority than most can handle.
(NASA bought these SUBS for their vibration testing since no other subs at that time could sustain the SPLs at such a low frequency and the high level tone bursts)
If you have the space, get two VMPS LARGER ($1400) and a great amp to drive them. Nothing could touch that (well only a couple)
Or if space is a limiting factor, spring for a SuperNova and never look back, knowing you have one of the best commercially available) subs for home theater on the planet (again among only a couple).
Boy aren't you going to have fun.
John Casler