When I sold my GK MB150S I switched to Markbass amplifiers and I find that they work well for double bass. I have a CMD 121P combo which is very powerful at 300 watts and weighs a hair over 26 pounds. For louder electric bass gigs I pair it with the matching extension cabinet for a 500 watt mini-stack.
I also have the somewhat less common Markacoustic AC 101 which I reviewed
here. This is a two-channel, 150 watt combo that weighs a little over 23 pounds. It was designed for acoustic instruments and sounds more transparent than the CMD 121P. It has some nice features that come in handy for the double bass and you can use the second channel for electric bass or a microphone.
For the ultimate in easy schlep-ability, there is the Micromark 801 which I recently got on sale at Musician's Friend ($275 with a President's Day coupon). This tiny combo weighs 14 pounds and sends 50 watts through an 8" speaker. The single VPF filter tone control is too limited for double bass, but when I use my Fishman Platinum Stage preamp with it a get a nice, stage-worthy sound. It's a great little amp for travel and practice too.
I also had a GK MB110 which was pretty nice, but I recently sold it to a friend after I got the little Markbass combo. I would have no hesitation in recommending that amp as well. I've also been curious about the Traynor SB 106 and I'd like to hear more real-world experiences about this amp.
I hope this information is helpful. Good luck!
- Steve