Gain knob doesn't work well.

Is is just the knob has broken (in which case, buy yourself a new knob and replace the one that's there) or is the potentiometer shaft broken too? That's a much bigger job, that involves pulling the whole amp apart and replacing the pot --- you need to get an electronics tech to do it.

Google
mg series knobs
to find replacements --- they just push on.

I would test by first trying a different guitar cord. If the fault is still there, try jumpering the effects in and out jacks with a patch cord. If there is sound now, clean the two jacks with contact spray and working a 1/4 inch plug in and out of the two jacks.

Are you using a single coil pickup guitar (Strat or Tele)? If yes, then this is normal depending on how much gain is dialled in.

"Overdrive Channel: high frequency noise as the gain and volume are increased."

This is the expected result, you are experiencing feedback. Try turning the "Contour" knob all the way to the left and turn down the tone knob on your guitar.

faulty lead? when the lead is plugged in turn the amp distortion on (may be called 'gain') and when you tap the end that goes into the guitar with your finger you should hear a buzz. try another lead if not, and if it still doesn't work make sure that volume is up on the amp and the guitar. there may also be a knob in the back of the amp, turn that up too (but not all the way or when you do get it working you'll blow the speakers out).

Turn down the treble gain and mid gain... you don't need it for a bass. Prevent the guitar from "hearing" the output of the amp directly. The guitar strings make a good microphone.

Sounds like one of the tubes needs to be replaced.

If the guitar can "hear" the sound, the strings can resonate from the sound and result in feedback. Running at maximum gain is asking for trouble. That causes distortion which drive the speaker as a square wave and worsens the problem. Also back off the treble as you can have supersonic feedback that you can't hear but will heat up your amp.
Test by allowing the feedback and then dampen the strings... if this stops the feedback you need to move the amp away from the guitar. Being a passive pickup makes NO DIFFERENCE about the problem.

You need to adjust the controls for the nano on the control cavity plate on the back of the guitar the way you want them first. Here is a link to a simplified explanation:http://www2.gibson.com/Products/Electric-Guitars/Les-Paul/Epiphone/Les-Paul-Standard-Ultra-II.aspx A better explanation should be in the manual but I don't think they did as good of a job as they should have.

There are 2 channels, Which means there are 2 sets of knobs ... One for clean and one for crunch. Start out with a low master volume and set up the crunch, Then the clen and switch between the 2 and make adjustments until they match volume wise ... Then turn the master up.