How do I know when to replace Melos SHA - 1 Tubes


I purchased a used Melos SHA-1 Pre-Amp over 5 yrs. ago, since then I have upgraded my Amplifier replacing the Rotel with the ARC VT 100 MKII..How can I tell if my tubes need t be replaced? I do listen to quite a bit of vinyl & I'm using the McCormick Phono Pre Amp. Should get a new PreAmp? Is there a Pre Amp that will work well with the ARC Amp which has a phono stage..Or should I continue using the Melos? How much for new Tubes; which tubes & where can I find them?
The rest of my equiptment is:
ProAc 2.5 Speakers
Meridian 518.20 CD player
PS Audio P300 Power Plant
hlt
Casouza, thanks for the tip on those Amperex/Philips tubes! Probably impossible to find, right?
You'll never really know for sure, it'll just always be in the back of your head, distracting and gnawing at you. The seeds of a conspiracy planted to induce paranoia, overtake your senses, and the world as you previously knew it.....or thought you knew it?

:-)Just kidding!...............................or am I? :-)