Is my DAC fried,cooked or what,help

HI to all AUDIOGONERS
This the problem I experience with my newly purchased (used AUDIO RESEARCH DAC-3 with four 6922 output stage)
It came with FEDEX a week ago and on last Sunday 18 July
I hooked it up to the rest of my system. I noticed right away a problem with the sound,lifeless,low very low volume level and pops coming through the speakers. I started a thread under amps-preamps thinking that it was my ARC LS3B line stage that was resting on my rack untouched for good two years. You see I thought it was a dusty volume control or dead capacitors or something.
I even called AUDIO RESEARCH and I was about to take my line stage in for check-up.
Today having a lot of free time and doubts in my mind regarding my LS3B I decided to experiment futher more and perhaps find the root of the problem.
First I went to RADIO SHACK and bought CONTACT CLEANER & LUBRICANT for switches. Came home and sprayed the volume control of LS3B really good.
Then hooked it again and guess the problem was there like always.
Then I decided to remove my digital front (krell transport and dac-3)out of the system and reconnect my MUSICAL FIDELITY A3 stand alone cd-player to my LS3B LINE STAGE and CONRAD JOHNSON MV55 .Wow problem FIXED.
No pops no low volume no nothing ,everything normal.
Furher experiment with another dac (STAX TALENT my old one)
and KRELL transport back into the system and line stage LS3B feeding the CONRAD JOHNSON MV55 proved to be excellent as well in terms of sound quality.
Therefore the root of the problem is the DAC-3 and not the LS3B line stage as I previously thought.
Now could it be that the four 6922's are fried and need to be replaced with factory fresh ones?
Or is it a more serious problem and instead of spending another (100$)for tubes I should check with FEDEX as the previous owner tells me so (I called him to explain the problem).
He was using TELEFUNKEN NOS which he removed prior of shipping the unit to me,he says it was working fine when he had it.
But if you see the tubes in the unit now have a dark ,black discoloration crown on their tops.Is this a shine of worn out tubes?
I need your opinions as I am not an expert just love good equipment and fine music reproduction. I do know how to connect my stereo and it is not a cable issue here.
If I take it in for check-up somebody has to pay for the charge and I sure hope it is just needs new tubes.
Anybody with tubes experience would know.
Thanks in advance.
Highly concerned at the moment.
George