I have been fannying about on this very topic - bid toget mandolin heard over violins, bajos and morris drums.....
Most of the belt amps are only 1 watt or 1.5 watts RMS at most. It is quite simply not enough. The only way to get my mandoin heard in the parade at Faversham ws to whack the little marshall up flat out, my belt-pack preamp up flat out, and dial in the overdrive distortion on the marshall too. That would have made a vioce wholly incomprehensible.
I think the AXL thin amp would be a lot better and I so nearly bought one on ebay but I was convinced it would go out of my range and then after I ahd not bid it didn't! THEy do 10w RMS but they neeeed a battery pack and the running time is a max of 3 hours before youchange batteries.
I am awaiting delivery of a Yamaha 5 watter (I may have to sort carry strap or belt clips myslef) and there is also a Yamaha 7 watter.
THe AXL is about 60 quid retail, alas, but I think it is likely to be the best for the job.