It really depends on two things: budget, any plans to upgrade to ATMOS?
If the answer is "I don't want to spend much money and I'm not ready to spend money on ATMOS": get a decent 2ch or 5ch USED amp on whatever local used market website you have (kijiji, craigslist, ebay, etc...). I'm sure in the SF Bay area you can find a Rotel RMB-1075 for under $500 or even some Emotiva XPA's for even less. Having the added dedicated power for your M80's will probably open them up too as your Denon is 105wpc?
If you have been thinking of upgrading, the equivalent new AVR-X3300W is about $1k and then you will need 2 new speakers for ATMOS and an extra sub :-)
Unfortunately, going Pre/Pro is expensive as I'm unaware of a inexpensive pre that can do ATMOS. Emotiva's XMC-1 Pre starts at $2500! and Outlaw audios pre doesn't do Atmos.
If you stay 5.1... get yourself used 2-ch amp that can to 300wpc :-)
