I'm running 20' RCA cables and it didn't make a difference in sound when I used a 3' RCA cable, I compared them in the same room with the same equipment. I'm not saying your balanced output may not sound better, but I don't think that going from 15' to 25' with RCA is going to make any difference, unless you are picking up noise on your RCA cables.
^Entirely likely as well.^
Which is why I first mentioned the star-quad with the ProFi ends.
And coax is also an option.
If that doesn’t work, then desolder and go to balanced.
But he stuff is only $1/ft or less so it is probably just cheaper to order it if doesn’t have a soldering iron.
Maybe you could try one amp (then the other) using an RCA double end male to hook your two shorter cables together…??
If that works with no humming, then just order the right length with RCAs.