When to change your amplifiers tubes?

I've had the same tubes in my amplifier (Audio Research VT100 MkII) for at least three (maybe four) years now (I'm guessing that I average about 7 - 10 hours a week listening time). The amp sounds good to me, but I'm wondering if it's just because I've become use to the sound. It's going to cost me anywhere from $750 to $1,000 to retube my amp, so I don't want to retube it unless it is time to. So here are my questions:
What audible signs do you hear that indicate it is time to retube?
Should I consider retubing just the power tubes, and wait to retube the input tubes?
Any thoughts you'd like to share with me will be greatly appreciated!
Is the $700-$1,000 for 1 amp or a pair? Either way it`s way too much.
Try Parts Connextion in Toronto, I used them for the past 10yrs or more, $450, for all tubes for a pair of Audio Reaserch Classic 120`s, 6550c`s Sovtek`s, matched pairs or Quads and drivers,

Never a problem or a dud.
I am not the authority but most of my gear,examble, my amps,pre-amp and digital converter are all tubes,so yes you are correct,it takes a lot of money to re-tube.From my own experience with tubes and from what I read on forums,I believe that by your description,you don't put that many hours on your amp,10hr per week for four years equals to 2000 hours.I believe a power tube lasts at least 4500 hours and maybe 6000 hours if not driven hard.The signal tubes last even longer.To prolong your tubes life,re-bias your tubes every few months (for examble in my amps is easy to re-bias so I do it often) turn the amps off when not in use for a long time and you should be fine.Some people own tube testers to confirm their tubes wear.I once had bad tubes in my DAC and it sounded like harsh,crackling noise coming through the speakers when the volume control was touched.I almost thought it was the pre-amp.
I hope this helps.
Popping. Cracking. And then my fuse clipped days later.

The driver tubes effect the sonics the most so try changing them first.

Spent $800 plus on 2 Quads of Genalex Gold Lion KT88/6550 and 12 Mullard 12AX7 types.

And for something different try EAT KT88 Diamond-Quad.