Does the preamp have a mute button or one of the tape monitor buttons pushed in? Sometimes stupid things like that can trip a guy. Is there an adjustable gain on the back of the preamp?
Also some source somponents may cause problems. Like Rich suggested, undo everyting but speakers, amp, preamp and 1 source. If this works, start adding things one at a time. If it does work, then tell us what's going on.
Wayner