I am wondering if the preamp upgrade for thousands more will provide any real marginal gain.
@jelliott78 The short and long answers both involve 'yes'.
Since your amps support balanced line, I would consider a balanced line preamp so as to get the most out of the cables and the amps. The balanced line system has standards in place that have been there for decades. Not all balanced preamps in high end audio support the standards. The standards are useful as they prevent coloration from interconnect cables and also prevent ground loops.
The main standard is known as AES48. You can ask the manufacturer of a potential preamp if its supported. The second aspect is the ability to drive a lower impedance such as 600 Ohms. This is harder for a tube preamp but they do exist. If you get a preamp that supports both aspects of balanced line operation you won't have to worry about what cable sounds the best ever again; to my thinking that's a real benefit!
Good Luck on the quest!