the smallest one I've seen was the JL 5.1 amp.. w/ spdif in... but its pricey...
Is there any reason you cant keep your current setup? or how about just trying to find smaller amps to replace yours..
The older (like 1-2yrs now) Blaupunkt PA2150 and PA4150 were really tiny and had good power..
And theres nothing wrong w/ having too much power.. as long as you set your gains properly you should be fine, cuz even though the amp is rated at 150wrms it doesnt mean taht its constantly pushing that....(only at max volume)