Yeah, I'm on the lookout for some decent monitors for the spare room too. I don't want to lug my main amp and speakers from the living room up there, after all.
Bear in mind, though, that Richer Sounds can come up with separates that will knock the socks off any active speakers, e.g. they're doing a
Cambridge Audio A1 (a very decent domestic amp) and a pair of Tannoy F1s (decent monitor speakers) for just under £200 all-in.
Worthy of serious consideration unless you really, really need the space, I reckon.
But if it absolutely must be active speakers, you'll be hard pushed to do better than M-Audio. They start at £69.99 for a
pair of Studio Pro3s (but I'm not sure they'll be heavy duty enough for you). They have some beefier products
here (note prices are in US dollars, so halve the price for pounds

)