Passive Bi-Amp using XLR-Y Adapters - Amp or Preamp?

I want to use two power amps for my speakers [passive bi-amping].One of the amps will power the left and right posts on my loudspeakers, and the other will power the left and right midrange/treble posts on my loudspeakers.
Since my preamp only has one pair[left and right], I will have to purchase XLR Y-adapters to make a pair of outputs per channel.
My question is:
For the least sonic compromise and the least signal degradation, would it be best to buy XLR Y-adapters that split the signal at the preamp or at the power amplifier?
Thanks in advance.