IMHO, I must agree with those who advise against using a guitar amp for bass. As was mentioned, most guitar amps are designed to give that wonderful overdrive/overtone sound without a lot of volume, so they will definitely start to break up and distort probably a lot earlier than you'd like for most intents and purposes. There are a few......perhaps very few guitar amps (mostly larger heads - - say 100 to 120 watts) which will provide amp/cabinet samples for clean output up to full rated power, but then this appears to go against your search for a small, light-weight combination. Here's what I'd suggest - - I just picked up one of the new MarkBass Micro Bass 108 units and am blown away by its power output and sound quality for its size. It's rated at 50 watts RMS into its included 8 inch neodymium speaker and weighs 11 lbs total. It also has an additional speaker output if you want to drive a larger cabinet. Also, if you are looking for "that tube sound," consider using a tube DI box or preamp with it. Best of luck with your search.