I'm pretty sure it's the former.
I can guarantee you this statement is correct (running out of amp).. I was powering my M60's with one the below amps.. well, the next generation, a CX.. But, the spec's are pretty much the same. The M60's never flinched, and got louder than i would ever want and sounded great the entire way up...
http://www.soundstage.com/revequip/krell_fpb300c.htmThe 300cx is what i have.
http://www.krellonline.com/assets/support/0162_020_MAN.pdfMy "usual" music listening volume is around 90db, in a space that is approximately 2400cu/ft space..
The amp is now powering the front drivers of a pair of LFR's...
Also, i have 2 EP 600's.. The subs will add a LOT of volume to the room, especially in the lower frequencies, for obvious reasons.