Cronus Magnum II Buzz

Greetings, I've owned a CMII for several years now, which I've been happy with. It gets light use (lighter than I'd like), recently I've encountered a harsh buzz when I turn it on, one that's coming from the unit itself, not the speakers & doesn't vary with the volume or selector. It goes off after a few seconds. More recently that buzz will start when its playing, last a minute or two, & then disappear. From what I've been able to gather this is a fault with the preamp tubes, the 12au7s or the 12 ax7s, but before I run off & replace them I just wanted to confirm this, & ask if there was a practical way to check them beforehand.

If you're my age you might recall those tube-checking machines they had at pharmacies & hardware stores where you'd take the tubes from the TV to check, replacing any that reported bad from stocks in the back of the unit. 

Pity they're not around anymore...

Thanks...