Pre-amp cables of different length? Or not??

I can't imagine that the ordinary person would hear a difference - however, since you seem to be willing to spend a lot of money to tune your system, you are probably not an ordinary person - are your really sure you have the ability to overcome the doubt you might have about the differences that might exist? If I were you I'd buy matching lengths, then you will never question your choice.

If it were me I would not buy cables of varying length. You strive as much as possible for synergy and balance in your system, but this would be one obvious way of throwing that balance off.

Are the amps monoblacks, are you bi-amping or are you running a multi channel system? Is they are monoblocks this would not require two pairs of cable. I'm just not sure what you are trying to accomplish.

Equal length cable is probably the best, and go for the shortest possible length.