Welcome to the forum, Tony!
This concern is precisely why receivers and pre-amps allow you to calibrate your speakers (automatically or manually). You can compensate for inherent loudness differences of your speakers by either cutting back or boosting the levels sent to them. Your new M60s will only be able to overpower your center channel if they are poorly calibrated.
What are your speakers connected to?
I run a Yamaha RX-V665 and an Emotiva UPA-5 amp. I do calibrate with an SPL meter. I'm having some issues with YPAO at the moment.
