I actually beg to differ. I enjoy my M60s with a sub (see my sig). I initially had M60 alone in a two channel Rotel separates system without a sub, and purchasing my REL sub improved the sound quite a bit, after tweaking it for a few days. In addition, adding a sub is a wise move even for a 2 channel system, in terms of an upgrade path for a future HT system, as it in fact happened to me.

Axiom M60s, QS4s, VP100 Onkyo TX-SR804 Oppo 970HD Rotel RB-1080/RCD-1072 REL Q150E sub, PS 3