Edit: Replaced the tubes. Still cut out or no sound at all when taken off standby. Check the fuses. They look brnd new. Bypassed the FX Loop and it seemed to work fine foa  day then went right back to cutting outor no sound when taken off standby. 

Full discalimer I don't have a lot of experience with tube amps.  Not many tech options to take it to where I live. I recently bought a used DSL20C and it is in imaculate condition.  Still has the tag on it. For the first few days it worked perfectly as expected.  I turned it on this weekend and let it warm up for a couple of minute, plugged my guitar, flipped off standby  and no sound.  Not even the usual hum that let's you know it's on.  I tried different guitars and cables. I confirmed and visible conections. Eventually, after switching the standby switch a few times it did eventually make sound. There were some pops and volume changes but it worked. I didn't press my luck and turned it off.  I took the back off and made sure the tubes were still seated properly (it was shipped to me).

This is the first time I used it with the speaker instead of the emulated line out. I really had to crank the volume to get good volume in my headphones.  I don't know if that's normal or a symptom. BTW the Emulated line out worked even when the speaker wouldn't. 

Does this sound like it need new tubes or am I looking at something more complicated.  I appreciate any help.