Sounds like a ground issue then, maybe worth just connecting a wire from the earth socket on the Lamm to a chassis point on the SS power amp to see if it drops the noise floor. Or try a different IC.
For me, too high a noise floor can interfere with the subtle harmonic decays that add to sound staging and involvement with the music, even if the noise itself isn't that obvious. And it is a great preamp, as you say, so worth trying to get the best out of it
