I think this really depends on what you're expecting in terms of LFE. When I moved and upgraded my HT, I had my M60s running without a sub for a few days, waiting for my SVS to arrive. Compared to the 80W Yamaha sub I had prior to my upgrade, the M60s did a rather respectable job for LFE. But the SVS or a similar high-quality sub is in a whole other league. The difference in bass between the M60s alone and the M60s + SVS (I have a 20-39PC+) is tremendous.
If you don't need room-shaking bass, the M60s without a sub(or M80s, as others have mentioned) are probably not a bad compromise. But make no mistake about it, it IS a compromise.
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