I'm considering adding an active sub in our ~18'x25' practice (aka jam) room sometime in this next year.
Acoustic drums, electric piano, plus a varying number of visiting firemen which usually includes electric bass and at least two plugged-in guitars (of various sorts), vocals in varying number/quality (usually 3x or more), whatever other instruments appears for a given session. I am able to match my drum volume to the mix, and I don't overpower the room... so we have no intention to mic any of the kit. (Although I may eventually get a wild hair up and maybe look into an electronic set some time in my life...)
Undistinguished PA: Mackie CFX12 mixer, Alesis RA100 amp, two Toa 12" tops elevated and facing toward musicians as "stage monitors," usually enough inputs available for all mics and all instruments through the board, plenty of power for the room and the mix, plus sufficient tone -- so traditional instrument amps are available but usually not required.
I suspect a 12" sub may be quite sufficient to complement the tops, but would benefit from suggestions, commentary, and debate. I would describe the idea as notional at this point, so the budget would be "open for dicussion" -- with value being a useful but not sole criterion. (I'm old, don't do weights well anymore, etc.)