Your Denon 4802 has more than enough power to drive the M60s at ear splitting levels. If you still need monoblocks, Parasound and Odyssey make highly regarded and very expensive monoblocks. The Outlaw M2200s are far far cheaper, but still considered very good. If you are on a budget, consider the pro-amps discussed here: :-)

http://www.avsforum.com/avs-vb/showthread.php?s=06b4076ce1ab0655b536ca03e293eb8a&t=418666&page=1&pp=30

4 M80s/VP150/EP600/Denon AVR-5308 & DVD-2910/
2 QSC SRA3622(1100wattsX2)/Carvin 1800HD(600wattsX2)