The M2 can indeed take an input from an external pre - well, at least according to the manual it can. It says: "FIXED: This setting should be selected when M2 is connected to the output of a preamplifier and is used primarily as a power amplifier. Output level is fixed and the M2s Volume Control is bypassed. Adjust the level using the source signals preamplifier volume or input level control."
Again, I think doing so would be to bypass the M2's reason for being - but it does at least appear capable of acting as an amplifier only. Using the M2 as pre of course isn't possible, since as you said Foster, it doesn't have the outs. I wish it did - would like to be able to connect a headphone amp to it.