Before jumping to the "go integrated" path, is there anything you do not like about the sound of your current pre/amp setup? If anything... ?
-Your M504 amps is no slouch. When they were still made fairly well.
-Why do you feel you need a tube preamp, maybe for a change in sound?
Not saying a tube preamp won’t help, more curious what you hope to achieve. A nice tube preamp might be fun too. Those M504 amps are kinda neat. Options.