Rodge, my welcome as well. Your room is smaller than average and as the previous replies have indicated, in no way are your M3s too small for it. As Ken mentioned, I'll also point out that at a comfortably loud average listening level(say about 80dB at your listening position)your M3s will be using no more than 1 watt, and even brief peaks are likely to use less than 100 watts. So, the "only" 95 watt rating of your 657 is likely to be more than you'll ever need.
A sub would give you significantly stronger bass below about 60-70Hz than do the M3s(an 80Hz crossover set by the receiver would be appropriate)and this would apply to low bass sounds occasionally present in music, as well as the separate LFE channel for special effects sometimes present in DVDs. So yes, you may find that a sub would significantly increase your enjoyment, and might be the most worthwhile purchase to make next. Incidentally, the power of the sub amplifier is only one factor in its performance, and isn't a very good criterion for making a buying decision.
