help! no sound

Before you had music. But now…you ain’t got Schiit. 
Sorry couldn’t resist. 

@mcanaday -- "Seems weird that would lead to NO SOUND rather than hissing"

Hissing usually happens due to a tube's age as it approachs the end of its useful power-on lifespan. That can range from 1,500 or 2,000 hours for a power output tube to many multiples of those figures for tubes used in preamp stages.

Obviously, a tube can fail in other ways, sometimes quickly, that completely interrupts the circuit.  Sounds like this may be your situation.

Hope you find the replacement tube fixes your problem.

Hello mcanaday!  Cables are the least reliable things in most sustems. Particularly the ones with phono (RCA) plugs used in most interconnects. You mention you were putting a system back together. Just this evening I was putting a system back into operation following a flood and I found two cables that were intermittant and had to be wigged jusst right to get the signal thru. I have lots of cables and replaced them. I am a very careful guy and these were from a reputable company. But they do suffer from a lot of pulling and pushing and go bad somtimes. Check them first. Good luck! 

I didn’t get that in the OP you were trying to use headphones.  I am also still confused what happens when you run the CD player directly into the amp with RCA cables.  Are you getting music or what?  If you are successfully playing music from the CDP into the amp then the problem is the DAC

I thought the OP was pretty clear that he also tried the RCA output from his CD player (which works fine into the speaker amp) and this gives the same no-sound results as using the Bifrost DAC. Definitely sounds like the Lyr headphone amp is the problem.