Good that you narrowed it down to the pre. Also good that the 555ii has the manual switching of sources (the listening control) because that switch is easy to clean (some later units had electronic switches, and anecdotically at least, those seem to have problems). The advice about cleaning those controls given above are the next logical step, but apparently have not resolved the problem. Did you open up the pre and spray directly on the switches/pots, such as Dr.Audio recommended above? Generally, if you hear something on the bad channel (static, intermittent sound, weak sound), it is usually a cleaning issue. If that is the case, I'd keep repeating the cleaning procedure.
If the channel is completely silent, I'd do two things to try to narrow it down further. First, plug your source into several of the available inputs on the rear panel (tuner, cd, video aux, tape in) and change the listening setting to the appropriate one, and see if the problem persists for all cases. These are all high level sources with similar input levels; the names are different just for your convenience. If the problem persists (most likely), at least you have eliminated the problem being on just one input circuit, and if that turns out to be the case, you can use the others and decide if you want to continue to chase the problem.
Second, open up the pre and look carefully around for problems, such as blown fuses, discolored components such as resistors or capacitors, bulgling/leaking caps, broken connections, bad solder joints, etc. Also if you don't already have the service manual, it is available for the series II unit over on hifiengine (
http://www.hifiengine.com/manual_library/adcom/gfp-555.shtml). You will have to sign up to download, but it is free. The service manual does have some layout and circuit diagrams, and a parts list, but unfortunately no diagnostic procedures.
