i've suffered through the same problem when i used to play small clubs in nyc in the '80s (max's, cbgb, the pep, etc). norman dunn, the former sound man at cbgb's, told me about the Tom Scholz Power Soak and I've been using it ever since. It's the least costly of the lot (though its hasn't been made in years.) you can find one for about $70-100 on the 'bay every now and then.
There are higher end units as well that are still being made: Marshall makes the Power Brake and THD makes the Hot Plate. I don't have experience with them so I can't say good or bad. maybe some other forum 'bro has used them and can chime in.
I eventually had an 18 watt amp built out of a Fender Musicmaster Bass Amp. It's one of the best investments I made. My amp guy basically turned it into a Fender Blackface Deluxe. Top to bottom it cost $650 which includes the $130 I paid for the amp on eBay and a new Jensen AlNiCo speaker. I added an ART MR-1 for reverb but other wise play it as is.
I've taken a few photos and will post later along with the specs. I know my amp guy likes to use the MM as a base since there's room in the cabinet and on the chassis to change/add parts. hope this puts you on track to fix your problem. --mike c.