New Tubes for HD150

Currently I have 2 6L6GC and 2 6V6GT in my HD150.
Looking at the back of the amp, the 2 6V6GT tubes are on the left and the 2 6L6GC tubes are on the right.
The 6V6GT tubes are Electro Harmonix Made in Russia
The 6L6GC tubes are National Electronics Made in U.S.S.R
First Question:
Can anyone give me an idea of the age and quality of these tubes? Obviously a "Made in U.S.S.R" tube is at least 24 years old...
Second Question:
Is it okay that I have 2 6V6 tubes in the left two positions and why would someone put 2 6V6 tubes in the amp when it calls for 4 6L6 tubes?
Third Question:
I'm considering a re-tube unless I have some really rare/awesome glass. If I re-tube, what tubes should I go with? (brand and model)
Fourth Question:
If I re-tube, can I just get a matched quad of 6L6 tubes and avoid a re-bias?
Thanks a ton!!!

Short answer to your questions is that there is no real answer. They have made tubes in USSR(I think) since then but the EH's haven't been IIRC. So yes, they could be that old. No, it's really not OK to have your tubes mixed. It obviously WORKS that way since you've been using it but it's radically conducting differently on both sides. It needs a quad of 6L6's in there. What kind? That question comes up all the time and honestly, those opinions vary as much as the wind. I prefer JJ's simply because I have installed them a LOT and have no failures and they sound good. The recent announcement that winged c is getting out of the amp tube business is going to make them even HIGHER in price than tey already are. Some guys like the new mullard reisssues, I haven't tried them but they are still coming from the same source which is the guy that bought the NAME, not the tubes. He's making tungsols too. Ruby used to be reccomended by Vintage Jon whom I trusted but that was a few years ago. Some guys have NO luck with them. No, you need to check the bias anyway because you were running the amp with two mismatched pairs of tubes and it NEEDS to be set for a matched quad. I guess I really don't have any good news for you. Sorry, just the truth here.

Thanks so much for your reply Mike! I was hoping to get an answer from you. Any chance the current 6V6/6L6 combo has done damage to the amp? I received it with this configuration. Also, if I'm going to have to re-bias the amp anyway, is it worth getting a matched quad or should I save money and not get matched?
Also (common question I'm sure), is biasing this amp pretty straightforward? I'm not a total newb. I've built a couple pedals and wired a few guitars. I've also had courses in circuits from a couple engineering degrees and I'm aware of the safety hazards found inside these amps.
Thanks!

You still need the match quad. The tubes are driven in pairs in that amp. Biasing is pretty straighforward. Check out the threads in the very FIRST thread in this catagory. Yours in the one with the SS driver. When you bias, you are setting the VOLTAGE across a fixed resistor to a minimum level with a maximum stated. Easy Peasy. Don't think it's hurt your amp if it's still working. Just not a good idea. NOW, there IS an instance where mixed tubes have been used. Check out the discussion on the eurotubes site about integrated quads. Pretty interesting idea. Mike.

Sounds like the integrated quads discussion got a little heated between the originator of the mixed quad idea and the PV folks. Even if it works for me and has worked before, I think I'd prefer to just get my amp back to running a set of matched 6L6GC's.
Thanks for your input!