Help with Belles 350a

Tape monitor switch engaged, not on proper input, muting on? Did you check for this?

"Listened to the source through some headphone(Audible
Illusions L1. All to no success.

So, I'm a little confused here. Did it work with headphones
or not? If it didn't, it has nothing to do with the amp.
The headphone stage is totally independent from the amp.

The L1 has a separate headphone switch that turns the output
on and off. Make sure it is out position.

I have no idea. Sorry. Hopefully we can help. How old is it? Do you leave it on all the time? Does it have the accessory ramp up power at turn on? I hope Almarg or somebody else replies. I have sent in my equipment into Dave Belles and he took excellent care of it. He seems to me to not be the cuddly type of man. He seems like a no nonsense type which I like. He has been friendly to me.

Like everyone said, you probably need to provide more info. I've seen people paint their wall outlets and the paint gets on the power cord or in the outlet and acts like a dielectric, but it sounds like you know better. You could have plugged a line level source into the phono input on your preamp, but I doubt that you would make that mistake, as well. If you really can't figure it out, I would take each piece individually, and try them in a system that is working.

I just thought of one other thing. You said you swapped IC's. Did you check the actual RCA connectors on your equipment. Sometimes the RCA connectors they put on cables fit very tight and can break whatever you attach them to. I've done that myself a couple of times. You have to check carefully, though. The connectors usually don't just break off. Most of the time the connection will crack at the soldier connection on the inside of the component. And if you have a partial connection, that could explain the muffled sound you are getting.