Your list of the equipment you presently have includes two preamps, one of which can accept a coaxial digital input as well as analog inputs. And your CD player provides a coaxial digital output as well as analog outputs.
So the system can be configured in at least three different ways:
1)CDP analog outputs to Sony Esprit preamp to SAE power amp to speakers.
2)CDP analog outputs to Nikko preamp to SAE power amp to speakers.
3)CDP digital output via a digital cable to Sony Esprit preamp to SAE power amp to speakers.
Have you compared sonics for each of these configurations? It seems to me that the existence or non-existence of differences, and their magnitude and character if there are differences, might provide some clues as to what the weak link in the system may be, from your individual perspective.
Also, after doing some research on all of the components you listed, while I certainly can't definitively pinpoint a weak link, I'll say that the SAE power amp and the Sony Esprit preamp seem like impressive beasts relative to the other components. And although on many occasions here I've cautioned people against assuming a high degree of correlation between performance and price, I think it's noteworthy that those two components sold for well over $1K in 1980's dollars, while the Nikko sold for less than 1/4 of that amount around the same time, and the Yamaha sells for $300 today.
Provided that you have confidence in their condition, and pending your answer about a comparison between the three configurations, and given your $1500 budget, my instinct would be to direct the investment to something other than a replacement for the SAE and the Sony. So in addition to the possibility of a speaker upgrade, you may also want to consider a CDP upgrade, including the possibility of one having a tube output stage.
And one thing I would most certainly NOT do is choose how to allocate the $1500 based on generalized notions of the relative importance of preamps, power amps, or other kinds of components.
Good luck, however you decide to proceed. Regards,
-- Al