Balanced XLRs to cable RCAs

Does your amp have a balanced/unbalanced switch?

Is the pin configuration correct & do both cables match?

Is there any difference in the materials between the RCA & XLR?

If pins are same from amp & pre, then cable should be fine, as they should not be sold as inverting unless requested. You can check continuity just to make sure there are no shorts.

What I meant by same materials is are they the same manuf? Silver vs. copper, plated vs. not, etc. will all yield different sound.

More than likely, some break-in required re: Sid42

Listen for a week (or 2) then put the RCA back in & compare notes.