Yamaha DTXpress/DTXplorer/DTXtreme

My 15 year old son has had a DTXpress I we picked up about a year ago. He
loves it. He has been taking drum lessons for a couple years. He has been
happy with headphones until now. I also have hooked up small self powered
computer speakers. He now wants to graduate to an external speaker. I am
looking to keep this somewhat affordable. Can I just use a home stereo amp
and speakers for a decent sound? Is there a good candidate amp & speaker
system I should be looking for on eBay or the local paper? While a home
stereo setup would probably be enough to fill the house he does hope to
play with school friends sometime so it would be nice to have some
reasonable growth potential in volume & quality although something
subprofessional is in order.