Musical ....
Fysascale ... A musical sub does things well like reproduce a kick drum that sounds like a kick drum, and bass guitar also sounds like a real person playing bass. A great home theater subwoofer handles DEEP bass (below 20 Hz), sometimes at the expense of great musicality.
Some people say "Bass is Bass" ... while I don't agree with that, it is said.
The ACI MAestro is one of those "magical" subwoofers which can do it all (it really is not magic, just a great design). You will get strong bass down to the mid to low teens, room dependent. It plays Stanley Clarke masterfully, and a Kick drum really sounds like one. And wait until you HEAR Lord of the Rings on a pair of Maestros ...
A pair of Maestros can be had, turnkey, for about $3900 ... A B4+, with the K1 Amp, decent EQ (a NECESSITY, not an option) will be around the same $$$ ... They will be pretty close in SPL ... You can add the K2 from Crown, and the B4+ will exceed the Maestros in SPL ... and, for the record, this was measured with the B4+ in two port mode, and with substantial EQ to 15 Hz...
The B4+ was up about 4 dB from 22 Hz and higher .... but at 15 Hz, the Maestro was up about 4 dB. This was with the K2 Amp.
SO ... for something that does EVERYTHING great, the Maestro, IMO, is the better choice. The B4+ IS a really good design, too. And this is YOUR money, not mine.
And yes, We have had several months experience with both ... you will hopefully enjoy the hunt !
Regards...