I would not say those Mullard RTCs are bad. They just don't seem to play well with LCD-2s. As I mentioned before, people with HD650s have raved about how great they are.
You learn what's good and what's not by reading, researching, but ultimately by trying in your own system with your own ears. The sound of a particular tube may be great to you, horrible to someone else. Every component in the chain of gear you're playing through can and will affect the sound and with the Lyr, tubes are a big part of that chain. You might love the sound of a certain pair, but then plug-in some different cans and find you suddenly don't like them anymore. That can get very expensive, very fast.
Personally, I think it makes more sense to work your way through different tubes rather than dropping the budget on super expensive glass out of the box. I've heard lots of people start off saying they want to buy the "best" tubes and that will be it - yet almost every single one of them still ends up buying/rolling others. Let's face it, tube-rolling is fun in this amp. It plays well with a wide variety of 6DJ8, equivalent and similar tubes, many of which are readily available, and it only requires one pair at a time for what can be drastic differences in sound. Along the way you can learn quite a bit about how to ID tubes, read date codes, spot fakes, etc. Again, taking that path is just my own personal opinion.
There's also a wealth of information in this thread if you take the time to sift through it.
Anyways, enough late night rambling for now... the most important things are that it's fun and enjoyable.
@R_Scott_Ireland likes the Mullard RTC's because he bought mine. Can't remember his pairing.
He also like the mid 60's green labels.
That said.... The sellers are trying to pass the white box Mullard RTC's off as the same as the green labels and they are charging way too much. They are not the same. No way 70's glass should cost over.... heck $125 ish.
Olga "frau_illa" (eBay I think) had quite a few Mullard RTC's at a reasonable price. There was a huge flood of these 60 days ago.. or so.
For the green label, mid 60's, be patient. Some NOS should surface for, I'll guess $180 to $225.
Edited by sceleratus - 7/12/13 at 10:38pm