What's a big loud head+cab I can get for $700?

[Gospel Bass Player]

The GK amps punch above their weight-class and the older RB-series have a snarl in the mids that really cuts through. They'll be more affordable too.

The Hartke LH-1000 that was recommended earlier is great sounding & inexpensive, but the mid EQ is cut-only, so IMO it wouldn't meet your stated criteria for low mids.

I use a GK 1001 RB (classic) 540 watts @ 4 ohms and have never cranked it past 12 o'clock...it is a beast !