Tube amp problem

I own Sonic Frontiers Control-1 amp. and just couple of days ago, i started
having a trouble turning the amp on. First day, i checked the fuse and it wasn't blown. When i placed the fuse back and started it, it worked! I turned on and off, couple of times and it worked fine.
Next day, the same thing. That time checking the fuse didn't worked so went through the whole deal of checking the tubes (removing them and trying to turn on), biased. It didn't worked. I placed the cover back and this time it 'fired up'! The only thing, i noticed was the strange static-like noise coming out of the speakers. I checked the speaker wires, but that didn't affect the 'static'. Few minutes later noise disappeared from one speaker than from another. I listened music with no problem for next few hours. Later, i pushed on/off and it worked 'till later that evening where it went silent. Again! Accidently i pushed the amp, and rear two 'feet' went of the stand, and amp turned ON! The 'static' came back but dissipated within the minutes. So each time amp doesn't come on, i gave it a 'slap' and it works!
Your suggestion and/or advice would be greatly appreciated!
Dragan, i expected the help from the Sonic Frontiers, but nothing. Yet!

P.S. Previous day before first trouble occured, i instaled powered Subwoofer via