If they are the same amps it should make no difference. Are you sure the polarity and speaker tap hookup are the same? Swap them back and see what results you get!
Swap Mono amps Left for Right
I had to disassemble my system and momentarily forgot which amp was previously left and which was right. I put them in swapped and I was much happier with the presentation. I think it may have something to do with my odd shaped room and maybe a different output level for the two amps? Not sure but I'm happy. Anyone try a swap just to see what would happen?