Hello,
I think I'll try it here, because I have almost the same problem:
I recently bought a Spider V 20.
First time I played it for some minutes, I had exactly the same problems as the OP:
* extremely loud distortion noise, building up to 100% feedback
* no knobs, including master volume did work
* I switched the amp off
Now, nothing works. When I switch the amp on, the "FX" buttons lit up in all colors, but only once. Then they turn off and the amp does not react to any input from guitar. No noise coming from the speaker anymore.
I AM able to connect the amp via USB to a Windows computer. This works.
With Line6 Updater I see Firmware 2.0 installed.
When I try to update to Firmware 2.0.1, it seems to work, but then Updater gets stuck with "waiting for the amp to reset". In this case the three FX buttons rotate blinking blue, but nothing happens. When I disconnect the USB cable and switch the amp off and on again, everything is as before - Firmware 2.0 is installed, but nothing works.
I tried to re-update Firmware 2.0 and even 1.00.3, with the same negative result.
Then I tried to use the "update mode", starting the amp with FX1 and FX3 pressed. This worked, too. The updater showed "Firmware 0.00" installed. I tried to install Firmwares 2.0.1, 2.0 and 1.00.3, but in the end of the procedure the update says "An error has occured. Please try again.", but sadly without any specific error message or error code (FX buttons still rotatingly blinking blue).
Is there anything I can do now?
I am a software developer and would be happy to use some more freaky diagnostic tools, if needed ...
Best regards
B.