Subwoofer REL dedicated line with amp?

Probably depends on the line...20 amps..30 amps or what? Also, probably depends on the particular REL that you own, as some have much larger amps than others.
I run my two REL’s off one line and my tube mono blocks off another dedicated 20 amp line...but my mono blocks are NOT wall sucking juice monsters ( I have a ss amp that is) and as such...there is no issue with running the mono blocks and the preamp off the same dedicated line. These days, one could question why anyone really needs wall sucking juice monsters, LOL.IME, some of the best sounding amps are low to mid power...at best.

Theory and results tend to differ with highly variable quality of household wiring and grounds. As others have said you are probably going to need to try it both ways...

i have

and with both stereo and monoblocks in my system with three dedicated lines each monoblock is on a line and the sub amps are on the third line.

IF you have an electrician out do a tuneup on your panel with cleaning and tighten connections, new high quality breakers, get the motors/ fans etc on opposite leg of audio

have fun