I've just received Re-0 bought here, it seems that I still prefer those M1 over Re-0, am I crazy or what?
M1 has very forward mids. RE0 has recessed mids in comparison and can seem overly bright and thin. I suggest a warm amp.
My first IEM was the M1 and 2nd the RE0, I also did not really loved the RE0 at first but with a iBasso T4 the RE0 won me over.
CMOY, Soundmagic A10, Fiio E7 with bass boost,... are all great amps to pair with the RE0. You could also try Comply foam tips.