I'm absolutely, positively not an expert (disclaimer, disclaimer!) but I would say that if you are in a largish room, go for the sub.
Why? Well, an amp is likely going to do nothing more than give you a bit more dynamic range. And, someday when you DO make a move towards a HT/surround system, you may not find much use for it, save powering speakers in another zone.
A subwoofer, on the other hand, CAN help give you a little bit more base authority on some music (This is the part where I¡¦m making assumptions, as my M60s haven¡¦t arrived yet and I¡¦m basing my bass assumptions on others¡¦ writings ƒº ). Anyway, a sub should ALSO give you a little more punch on your M60s as it whisks away all those watt-hungry frequencies to a dedicated amp and driver, freeing the watts you have to just handle the upper bass, mids and highs. Besides, when if you ever do go ¡§surround¡¨, you¡¦ll have a sub that¡¦s all but necessary with movie soundtracks.
So, in short, I think a sub will give you the same little increase in dynamic range as the amp, but maybe provide some frequencies you don¡¦t always get in full with your M60s, and be more ¡§future proof¡¨ as well.
Of course, none of this may apply if you only listen to solo violin concertos, have a small room, or if the M60s really DO sound as good in the bass as many say. YMMV. ļ
