Gibson EDP hiss noise

[Laurent Brondel <laurent@megalink.net>]

I am getting a modulating hiss noise on a Gibson EDP when I record. I 
set the output of my preamp at -10db and the input knob correctly (a 
tad before the red led appears) on the EDP and get this modulating 
hiss, like a bad compressor/gate would do. If I set the output of my 
preamp to +4db, and the input knob correctly, I get a mildly distorted 
sound coming out of the EDP, even though the led stays in the orange. 
Obviously I tried changing leads, plugged a guitar directly into the 
EDP, all with the same 'shrunken' sound and modulating hiss coming out 
of the machine. This is driving me crazy, the sound -besides the hiss- 
is not quite right. I don't think that EDP exhibited that problem 
before. I have another EDP -an earlier Oberheim- that sounds fine. 
Anybody has a suggestion, a quick fix, should I consider sending it for 
repair?
Thanks in advance,
Laurent