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