Hey!

I actually bought a used MP566 but only got to use for a few days. Now when I start it up I can see audio going into the preamp (you that little LED) but nothing is heard or being fed into my interface. 

I did change tubes from the JJ 12AT7 that came with it into a Sovtek 12AX7 and that worked beautifully for maybe 2-3 days. I have tried to switch back but it does not change this behaviour. 

Any tips on where I should begin looking?