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Strange amp problem

If you can't solve it by cleaning the efx loop jacks with contact cleaner, than you can use a short 1/4" to 1/4" instrument cable and connect the efx send to the efx return. This bypasses the signal path that normally exists when the jacks are empty, and simulates an efx loop.
 
It sure sounds preamp related if the efx return and on down the signal chain works well. I would try to isolate it if you are comfortable. Its a combo, so you will need to pull the amp out of the cab and get it up on a table so you can work on it. I would start by trying to bypass the input jack by connecting an instrument cable to a couple of alligator clips and see if there are leads off the back of the jack to connect to. You will need to hook up a speaker to the combo amp, but then you can play your bass and see if it bypassed the issue. If it does, replace the jack. Anything more than that and I would take it to a tech. If you aren't comfortable, send it to a tech.
Wes