I know my tubes (VT100 mkII) are going when...

I'll begin by saying that I do realize a lot has been discussed on this subject. I read a lot of threads here.  Here is my situation:

1.  ARC retubed and did some other maintenance on my VT100 mkII in April 2015, so they are 3 years and 3 months old.

2.  I have no Idea how many hours are on my tubes in my power amp or preamp.  Tube timers came in later models. Darn, I want a tube timer! (Side note, anyone know if there is a good external or accessory tube timer to buy?)

3.  I learned to check bias on the power tubes, but have not attempted to measure the signal tubes yet.  I am ready to try. I have the tools (need one more multimeter) and I like to be hands on.  When I checked the power tubes 2 weeks ago, they were perfect, despite not being checked or adjusted since ARC did the service in 3/2015.

4. I could be crazy, but I think my system is not as loud as it once was. I am not sure about a diminished sound quality, but it is possible that has occurred very slightly.  No noisy tubes or bad tube static though.

5.  Loads of people spoke about installing KT120s in this amp. Are people in agreement on this?

6.  Is The Tube Store reliable for well matched tubes?

thank you everyone!
- Mark