I'm reaching back into my memory here, and I may have this wrong, perhaps an engineer can help, but many solid state preamps have comparatively low output impedances, while tube amps have very high input impedances. Normally you would want the output impedance to be higher than the input impedance. This may not make a difference with short cable runs, but with longer ones I recall reading you could lose some of the high frequencies. I like Sugarbrie's suggestion to ask Bill Thalmann.
