Balanced cable end at which end?

Your question is unclear. A balanced cable will have an xlr connector at both ends.

Unless you are saying that it is an xlr-to-rca adapter cable, with an xlr connector at one end and an rca connector at the other end, and that the preamp and power amp give you a choice of which connector to use. In that case, the choice would be dictated by the gender (male or female) of the xlr connector on the cable (unless you add an adapter to the adapter cable, which would not make sense).

Keep in mind, also, that most xlr-to-rca adapters and adapter cables ground the "cold" signal on xlr pin 3, and some output circuits cannot tolerate that. See this thread for an example. That is not a concern on inputs, only on outputs.

I don't think that the fact that you are using a phono cartridge as your source, apparently into unbalanced phono inputs on the preamp, has any relevance to how the preamp is connected to the power amp.

Regards,
-- Al

OK, I understand now. Yes, the same principles would apply regardless of what components the cable is connecting. The end of the cable that is "fully earthed" should be connected to the OUTPUT of the component that is driving the cable (in this case the preamp), and the other end (the one that is not "fully earthed") should be connected to the INPUT of the component into which the signal is going (in this case the power amp). That will result in best noise performance.

Best regards,
-- Al