You could run one of those little powered subs that is intended to bump up a vocal powered PA speaker. Sub needs a line level crossover and combo needs a line level loop.
Line out of amp into sub, high crossover out of sub back to line in of amp. May work really well or may be loop buzz city.
Team Trace Elliot #1, Mediocre Bassist #399, Old Basstard #86 Kala U-Bass #22
Swamp Kauri custom 5str. Stagg EUB. Krappy 5er FL.