A P.S. to my previous post: If you do eventually have an opportunity to try a tube (or other) preamp in your system, and if you find the resulting sonics to be preferable to no preamp, keep in mind the possibility that the root cause MIGHT be a less than optimal tonal match between your power amp and your speakers. Particularly in view of the impedance characteristics of the speakers (4 ohms nominal, 0.8 ohms at 20 kHz), which could conceivably result in an over-emphasis of the upper treble in conjunction with some solid state amplifiers.
Should that ever prove to be the case, changing power amps would probably be a better solution than adding a preamp.
And if there is any uncertainty in that regard, researching reported user experiences involving the use of Spectron amps with Martin Logan speakers may be helpful.
Regards,
-- Al