You said that both switches are popping.
Let's try this:
plug the guitar in. left all other jacks out. plug in power jack.
press tails switch on and off.
insert out jack within those 20-30 sec when you had the popping.
is it still popping?
If you have 2.2u (may use 2 in parallel to increase capacitance) or 4.7 film capacitor use it instead electrolytic for C20. Just for testing purpose try to swap C20 with the biggest film capacitor that you have. The popping should be gone.
read this
http://www.muzique.com/lab/leaky.htm
Also measure DC on output while its popping. How much is it?