My main little amp is a Cali Blonde. I have not used it with URB, but know of folks who have with great results..but I do use it with my piezo equipped Godin acoustics/electrics. It sounds great. I do run a littel SWR 1x10 extension cab, which ups the output and thus the headroom. Very nice combo, light and good sounding. I usually run my A/E's into a Raven Labs MDB or PMBii and send that into the Blonde via the fx return jack, thus bypassing the amps preamp stage, but running straight in also sounds very fine.
I have at times used it with a powered extension cab in stero, and this really sounds amazing...
I opted for this amp mainly for the portatbility and really neutral coloration of the amp. Mostly I use amps onstage just as a personal monitor and rely on the PA for feeding the house, and this little baby works great (I opted to get a ATA Case on casters for it, so it makes hauling even easier!) Having a mic input on the 2nd channel is a plus for small solo gigs and clinics!
I am realy quite surprised that more bassist have not "discovered" how wonderful this amp works for basses; even though SWR marketed it as an "acoustic instrument amplifier" it is one heckuva bass amp, and IMHO sounds much better than the WM12 or 15.
Max