Candela has relay controlled input selector. Are the relays clicking when you change inputs?
Did you look inside for anything obvious like loose connection, burnt parts, etc? Don't open it unless you know what you're doing.
Source and power amp are still working, right? Murphy's Law sometimes has a really bad sense of humor. 
Next step will be to probe the signal thru the pre. The first place I would check for signal is just before the muting relay after the output stage.
Are you hearing any tube hiss in the speakers, or dead silent?
Thanks for your response. I do hear relays click very shortly after switching sources. I did look for obvious problems under the hood when I swapped out tubes. I didn't see anything that stood out to me, and I have experience refurbishing a few pieces of vintage hifi gear.
I did indeed check to make sure amps and sources were all functioning, but hey, if no one asked...
When I switched to the CD input I could very faintly hear the CD coming from both speakers when I put my ear very close (within a few inches) of the tweeter/upper mid drivers of my Legacy Audio Classics. I do
not hear any tube rush or other noise with the preamp on and no signal.
I did look up specs for the Candela and my Parasound HCA-750A amps (which I'm running mono).
Candela's output impedance is listed as <2k ohms
Parasound's input impedance is 33k ohms Sensitivity is 0.775V
So it doesn't look like there's impedance mismatch either.