help! no sound

I have a Lyr 3, not a 2, but have you checked that the RCA cable is plugged into the inputs and not the outputs? And I also assume you've checked that the power cable is firmly inserted at the back of the Lyr and the cable is plugged into a live outlet at the other end, and the power switch is on. Sometimes it is easy to miss the basics.

@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.

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.

The OP had a second post that provided more information. That's the one where he said he used the RCA outs of the CD player to his speaker amp, which works without issue, and runs the CD player's coax out into his Bifrost DAC which then fed the Lyr, and didn't work. In this post he explains he did try the CD player's RCA output directly to the Lyr with the Lyr still not producing sound.