what is considered the most musical powered sub?

Looking for one that offers both high and low inputs, auto signal off/on and compact size.
The functions and qualities you describe are all combined in the Martin Logan Depth. About $1300 used.
Sunfire Architectural Signature. Front firing with a small foot print.
To close your bank account try the Wisdom Audio 7b subwoofer. Active or passive this sub does BASS very musically! Try active if you can afford it!
I've got a pair of Rel Strata III's which I use with
Magnepan 1.6QR's. The Magnepans are notoriously difficult
to integrate well with subwoofer(s). The Rel has a
highly variable crossover, high & low inputs so it can
be used simultaneously with both music and HT systems,
and tuneful well-defined bass. They are no longer in
production and are highly sought after on the used market
(I had to import my second unit from a private owner in
the UK). Recently two showed up on Audiogon in the
<$1000 range. It's an excellent subwoofer.
I have high regard for the REL subs. Very musical. I own older Vandersteen 2W subs, which can be obtained for about $700@; very reasonable to do a stereo pair.
They are sligthly different in that they utilize a crossover between the pre and amp. Well worth your time reading up on them.
if you go for a velodyne dd series (the dd10 is quite compact) you'll NEVER be able to go back to a sub that doesn't graph your room's freq response and eq the problems to fix it. The velodyne DD series of subs is one of the most important and revolutionary audio achievements in decades.
The TBI subs I have found to be excellent for music.