GK MB200 or Aguilar TH350.
I bought both of these in search of small set-ups, and both are excellent.
GK MB200-- someone once described it as the swiss army knife of amps, and that rings true. It's a back-up amp that you can have in your gig bag (it fits neatly into the zip pouch of a bass bag) and it occupies next to zero room, and weighs 2 pounds. Great for any small gig, and if you have two cabs that will therefore run at 4 ohms total, then you'll have enough speaker to get pretty damn loud for medium gigs. Makes a good upright bass amp. Great for going to rehearsals, practice, etc.
Aggie TH 350-- it's a little bigger than the MB200, so it doesn't fit in the zip pouch of a guitar bag, and a little bit heavier (3.5 pounds instead of 2.2 pounds), but it has more power, and you can get a very full, loud sound even running one 8 ohm cab (used it with a Barefaced Big Baby 2 with good results). When I say "bigger" it is still small-- but bigger than the tiny MB200.
Key word I heard was "back-up" and "small to medium gigs" and GK MB200 is perfect for that. Can get one for $250 new if you shop a bit.
TH 350 is not quite as super-portable but still very portable, and closer than MB200 to being an everyday, every gig amp which TH 350 basically is, with proper cab(s).
Finally if you just want another full-on amp for all purposes, to use as a second, then you could go bigger and stay in the budget with a TH500 or MB500. Personally, I have a "main" amp and was more interested in having a serviceable back-up/small footprint/difficult load-in set up that can go anywhere any time, and both the MB200 and TH350 do that.
