Has anyone developed a workaround that solves signal degradation when splitting. The problem that I have occurs even when the preamplifier has two balanced outputs.I think @cleeds got this right. The impedance of the two inputs is a bit much for your preamp. especially when the ISOMAX is involved. One solution would be to have a preamp that supports the balanced line standard (many 'high end balanced' preamps don't). Some of the ISOMAX transformers present the preamp with a rather low impedance. Unless the preamp is really designed (which, if its balanced, it should be, but regrettably most high end audio manufacturers were simply jumping on a bandwagon with their balanced entries...) for the low impedances which are common with balanced operation, its likely to fall flat on its face when presented with such things, as you've described.
Years ago I became aware of this problem when using my Sonic Frontiers Line 3, which has 2 pairs of balanced outputs, to drive a Theta Dreadnaught amp and a pair of Entec LF-20 subs. I thought the sound was excellent until oneday, I disconnected the cable to the sub and that’s when I heard what I had been missing from the Wilson Cub I, transparency and reverb that allows you to hear deep into the music.
A different ISOMAX **might** solve the problem if its able to present the preamp with a higher input impedance. But that would still be in parallel with whatever the input impedance of your amplifier is.
I run a similar setup but my preamp doesn't care about the low impedance loads involved.
