I haven't heard M1s but do own Atoms (used; the price was right) and have read a bunch of reviews on the M1s. As standalone speakers the Atoms are pretty nice -- they play loud and the bumped-up bass from 100-200 hz helps them sound more substantial than my M2s.

On the other hand the bass bump also adds some coloration that starts to get annoying when you A/B with M2s. After some testing I replaced the Atoms with M2s for stereo listening and put the Atoms in as surrounds until I have budget for QS8s.

From what I have read the M1s are more like typical Axioms -- flat response rather than bumped up in the bass. One could argue that the Atom sound is more like the M3/M40/M50 "mellow" sound and there probably is some truth to that, but there is more bass emphasis on the Atoms than on the M3s for example... and M40/M50 seem to be quite flat in the bass region.

It's a tough call re: which will match your 50s better. I would lean towards the M1s. On the other hand, if you're likely to want to use these speakers standalone in the future (eg. you get different surrounds) the Atoms are probably a bit better as "all round" speakers, the same way that M3s are felt to be better standalone speakers than M2 because of the slightly fuller bass.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8