A higher level than Mullard in tube quality?

I am just getting started in tube rolling which I find pretty exciting. Any thoughts or comments would be appreciated.

I have been upgrading the pre-amp tubes in a DK Design Mark II intergrated amp. My speakers are modded Magnepan 3.6's. My source is a Resolution Audio Opus 21. I listen to mostly acoustic jazz and vocals.

I went from a set of Electro-harmonix 6922 (like $20 each) to used 6dj8 Amprex "Bugle Boy" ($40 each) and that was a nice improvement -- well worth the cost. I just upgraded to Mullard 6922 NOS ($90 each) and WOW! I am amazed at the improvement. I could not have imagine that I could improve a $10,000 rig so much for so little. Now I am thinking ... what's next? Can I spend $300 and get another step up?

At this point, what is my next level up? Or is it more sideways to different type of sounds?

Question: used tubes of the same vintage are significantly less than NOS. Is there any down side to buying used (but tested) tubes? I can get 2 for the price of one NOS and probably get more hours from 2 combined. Also, it seems like a good way to lower the cost of testing different types of tubes ... or ... does the aging change the sound.

Question: I have Mullard Blackburn plant made 6922/E88CC gold pins in GE label from the 1960's. Would I get a different sound if I bought different Mullards, such as from a different time or a different plant? And do I just have to take the dealers word for the details -- it all seems so vague.

Right now I have $300 burning a hole in my pocket to try the next step up. But I fear I may hit the diminishing return ...

Thanks for your comments
While I prefer Telefunken and Siemens CCas to Mullards in my Jadis equipment, that is my personal taste and has nothing to do with the quality of the brands. I think all you're doing is changing the sonic flavor; by using NOS tubes you're also getting better-made tubes, so they should last longer. If you like the Mullard sound, keep in mind that it is different than Telefunken's and Siemens', and you may or may not like them better. I'd say stay with what you've got, since you like the sound, and spend the $300 on some music.

Aging changes the sound slightly, it'll get duller and less dynamic, but if they test strong it's worth getting used ones, at least for spares.

Answer to your last question is probably yes, but I wouldn't obsess over it.
Read "Joe's Tube Lore". There are many flavors and many variants of any tube. Sure, some are at what you "higher levels", but it usually comes down to personal taste. There are many archived threads detailing others' experience in their gear. Try searching for experience w/the same electronics that use will using. Cheers,
Just be careful who you buying the tubes from...Sometings tube testing and rating of ones tube can be pretty vague..I would spend the extra $ and go with reliable tube sellers...... vs..Joe Shmos' used but test new tubes.....
I had Mullard gold pins (NOS) in my Steelhead, but swapped them out for Telefunken 6922s (NOS). Sound went from warm and darkish to neutral and not edgy. A big improvement in my system, given my taste, although not inexpensive.
I always thought that Philips SQ 7308 were the best of this bunch and they go for around $200-300 on eBay. I've had the best luck buying from sellers who use good testers and post the results.

Be sure to ask first that the tubes are not noisy or microphonic and what the return policy is.
Different brands and vintages of tubes have a different (not necessarily better) sonic signature. Mullards are "warm", Telefunkens are "neutural", etc., etc. Obviously you like the warm sound of a NOS Mullard. Learn your tube codes (i.e., the Blackburn plant will have a "b" followed by a date code and usually an xf1 or xf2 on the tube). You do well to invest in a quality tube tester (find them through antique radio clubs, retired tv/radio guys, etc.). A warning though: KNOW who your buying from. I have purchased several "NOS" tubes that supposedly tested well only to find them with shorts, grid leaks, or nearly totally worn out. Usually when I present the seller with the test results they take them back or make good on the deal, but if I didn't have a tester, then what? Andy Bowman at Vintage Tube Services is reputable. Member JSautter is also reputable, as is Brent Jesse Recording, just to name a few. Good Luck and Happy Listening.
Just a note...

XF1 & XF2 codes have nothing to do with Mullard 6922's.

XF1/XF2/XF3/XF4 codes were used on different versions of the EL34 tube (a portion of which were manufactured in the Mullard Blackburn plant, as well as plants in Holland/Japan & perhaps other locations).
that $300 could go toward a pair of US made Amperex orange label 7308s, or even the white labels. Each will give you some of the midrange warmth of the Mullards, but with more air at the top end and a sharper soundstage.

I personally think it would be just fine to buy used tubes (especially 6DJ8 family tubes which have 10k hour life) as long as it was from a reputable seller as has been already mentioned here. You will certainly save a ton of money if nothing else.
If you want to test your own tubes and also check for noise, buy the Vacuum Tube Valley tube tester. It also tests at real world voltages.
If you're happy with those NOS Mullards, then just sit back and enjoy them for a few thousand hours. At some point down the road, try to find a good set of early 60's Siemens CCa tubes and give them a try. If the Siemens don't suit your taste, then pop those Mullards back in and enjoy them for another few thousand hours. In the meantime, if it's the Mullards that you enjoy the most, find another spare NOS set and tuck them away in your tube stash to eventually replace your present Mullards when they wear out. Enjoy your system.
It's not a question of higher quality. Mullard, Siemens, Philips (Amperex), Telefunken are all of essentially equal quality, but they possess different sonic signatures. The issue is one of sonic preference given the listener's system components, speakers and personal taste.

All are excellent tubes.
Thanks for so many great comments and tube suggestions. I read the enlightening "Joe's tube Lore", thanks Sbank. I think I will call Andy at Vintage Tube Services and see if he can put together a sampler of used (inexpensive) tubes for me. Then I can listen and compare the sonics in my system to see where my personal taste leads me.

My take from the comments here is that tube selection is kinda like cords and cables: you reach a level of quality once you get past the mass market stuff and then it becomes character differences and how the equipment interacts with the other components in your system -- and your personal taste.

I listen alot -- several hours a day. I am enjoying the improvement and my system now sounds better that I ever imagined it could. So I can (and do) sit and enjoy. But on the other hand, I am kicking myself for not upgrading my tubes a year or two ago. So little investment for such a great return. But I do feel I have reached a level that is pretty close to the best it can be so it is just tweaking from this point.

Now, given how much I am enjoying tube sound, I am wondering if I should move to a tube DAC. But that is a another post ...
All are excellent tubes.
. Yes, I think, that's the point. It is always depending on the System in General (analytic, cold/warm etc.) and how the unit works with it...
I have Mullards, Telefunken, Jan Phillipps, GE and so on here and in one unit one of these Tubes is simply amazing and exactly that Tube in another unit looses it's Magic.
In general I like Mullards (in the ML 2.1 it is a Killer Tube) but you will not go wrong with others.
So Thom, it looks like you have to update your system profile and pic to reflect the ML 2.1 and I would love to see a report on how they sound, now that you have had time to get to know them.