Interesting requirements.
Lightweight generally means more money - unless you search long and hard for something used.
Affordable - meanings under $1k.
I think the best bet is the Markbass amps. I understand the problem of having it serviced in the USA - so you only want only USA equip - that makes sense - however...
At 6.5 pounds I travel to gigs now with my amp head in a laptop bag. At 500 watts @4ohm - it has plenty of power for small to medium gigs. Ive even down an outside winery gig with it. I got it used for $500.
Some comparable USA models are GK, Claraus and the Eden WT 550 - each has more weight that the markbass. There could be other USA models available - but this is want comes to mind right now.
I think the small, light and loud - and high quality are a universasl requirement by a lot of gigging modern bass players. So check this link out:
http://www.youtube.com/profile_video...rkbassAmps&p=v
I hope this helps.