Another way to get a handle on Tube Research is to check out the Bluebook service here on audiogon. Anyone serious about buying used products should be enrolled in this service.
Yes, this is a subscription service, but considering how much one can save with this knowledge, it is well worth it.
I do understand that Tube Research makes excellent equipment. The new purchase price is very high, while the bluebook numbers show more than a 50% decline in value in the used market. If such data matters to you, you might look to McIntosh or one of the other products that seem to hold at least 60-70% resale value.
I have been watching the occasional Tube Research Amp pass through the classifieds, and I think that at the used price there is great value.