Considering your budget, I'd recommend a lightly used NAD C275BEE. The Ncore Hypex NC400 is getting great reviews but its a little outside your budget at $1500 for a pair of monoblocks. Probably one of the best class D amplifiers out there according to the reviews.They do need to be assembled. For the record, I haven't heard either one driving Magnepans but I have owned the smaller Magnepan MMGs and my experience told me the more power they get the better they sound. Buy the most powerful amplifier your budget allows and save some money to put a tube preamp in front of it later if it sounds too bright.