Carlsbad,
tksteingraber's advice to contact Paul Hart is spot on. He recalibrated my Hickok 600 about 2 years ago and did a fantastic job and gave me great advice to boot. Even if you're going to do refurbishment and recalibration yourself, he's a great resource.
My system is pretty much all tube, and owning a tube tester has made that so much easier, and allowed me to take chances on cheap untested tubes that turned out to be NOS testing jewels. The Hickok greatly increases my joy in this crazy hobby.
One caution based on experience: you can sell tubes that test great and you know are perfectly functional, and the buyer will still hate them because they were lousy in their system. Refund them cheerfully and chalk it up as a cost of doing business. Most will love them and the honest seller goodwill is invaluable.
Best of luck on your journey. It's worth it.