How long can I expect tubes to last on a preamp or MW CD player?

Secondly, what would be the signs of tube deterioration?  Without a tube tester, is there any way to tell how much life they have left short of they aren't working anymore?  I have 7 tubes between the TRL DUDE and MW 5400 Signature Truth.  I may have hit that point on at least a couple tubes.  Seems to have dropped a little in SQ.  The 2 SNL7 tubes on the MW are only a couple months old.  The rest have been on 24/7 for at least half of their lifespan, about 4-5 years with NOS tubes in them.  I still have the original tubes with very little use.

I just re-read your OP.

You can switch l/r, any volume balance change?

You can put the original tubes back in, different volume? if obvious, get some new ones.

Thanks for the advice e.  I will try that out.  I have that record, not CD, however.  I will say that the system has been leaning slightly towards the left with most all voices.  Didn't used to do that.  I have the TRL DUDE preamp so it will takes a couple days to make sure it's totally without any saved up power in the caps--I like to live, before I switch any tubes.  I can't really remember what tubes are inside other than a pair of SNL7's.  I'm not sure I have changed the other 2.

the tubes you are referring to is the 6SL7 gain tubes in your dude

try swapping them side to side see if the channel imbalance switches

call or email brent jessee or jim mcshane tell them you need replacement tubes for your dude - they will know what to send you (google search will give you their contacts, they are stalwarts in audio tube supply...)

good luck
I had a volume balance problem, perhaps it is your tubes.

Turned out to be one of my Cayin A88T's 6sl7's (it also has 6sn7's).

had several used GE 6sl7 downstairs, that proved it was just the tube, not the amp or anything else in the chain. 

bought new 6sl7's from Brent Jesse.

Picked a Sylvania model because the chrome tops would look different from the 6sn7's (I had mixed them up once).

Sylvania did not sound as good as the unidentifiable Cayin tubes, or the used pair of GE's.

Brent had me break in the Sylvanias for 60 hours (no signal needed, just leave then on). Still not as good.

I picked alternate GE, I paid postage only, Brent gave me full credit, and, the GE's I ordered and love were half the price.