Tubes lose their luster and/or get noisy when they are ready for a change. Unless you are trying to change the sound in your preamp I would use either NOS Sovtek6922's or 6H23EB's. I find the current Sovteks a bit too cool. If you want a different sound tell us what you want to do - someone will have a suggestion.
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Sounds like you could use a new tube. The 6922 has an average expected life of perhaps 7-10K hours, but this will depend upon what you are driving with them. Your LS2B does not tax that single tube much at all in my experience. My friend had a stock LS2B where the tube (I think it was a Sovtek) did not go bad for eight years. He travels a lot though so probably the unit had light use. Using the pre regularly anywhere from 2-3 years might be normal.
Upgrades: There are some great NOS tubes out there that you can spend plenty of money on. Since you only need one you can likely find a deal on someone selling a solo. Since my friend has a stock LS2B and I have a modified one we've done some side-by-side testing and tube rolling together. As expected the better tubes were the NOS Siemens cca's and the Telefunken. Mullard and Amperex Holland did not make the cut. My friend's now hooked on those Russian Rocket tubes that are a more hearty replacement. Kevin at Upscale seems to also like those as I've seen many happy posts from folks taking his advice on them. I think they are 6H30's. No personal experience there, but I do respect my friends opinion and he likes the results in his stock LS2B. As far as more expensive upgrades go, I can highly recommend the upgrades by Steve Huntley at Great Northern Sound. My LS2B was upgrade by them and the difference when we swapped out the stock unit in my friends system was immediately apparent: better focus and imaging, slightly wider and deeper stage, and improved clarity. Both of us agreed it was worth every penny. I went for the Standard Level upgrade which was around $800 as I recall. He also offers a Reference Level for almost double the price. Steve used to work for ARC, and then went over to Wadia and worked there for some years. He now owns GNS and does some highly respected modifications on components from both monikers. You can check out their basic website here. No affiliation, just a happy customer.
I tried to post an addendum to my original response, but it has not turned up, so I'll try again:
I'd have to agree with Newbee; the Sovtek 6922 is rather cold and I would add, a bit 'narrow', especially when compared to the German NOS. The latter are more warm, expansive an give a fuller sound with a bit more weight and focus. This occurs to varying degrees depending on the specific tube. The Sovtek Rocket tubes are only around $20-30 per tube, while the better NOS tubes in this family will easily be double that price. Again, since you only require a single tube for your LS2B you may find someone selling a solo at a good price (matched pairs/quads tend to bring a premium for that service).
Also, be aware that there are many counterfits as well as many relabled tubes in this common and widely used and coveted tube family. The latter is not necessarily a bad thing, while the former (fakes and counterfits) usually is. If you are not an expert it may be best to simply purchase from a reputable dealer like The Tube Store (my choice), Upscale Audio or Vintage Tube Service.
Thanks a lot guys!
I looked into the Great Northern site & like the it a lot & will likely take advice/purchase tube from them.
First I need to isolate my system to see what's causeing the H/F loss (at least an octave) as well as lack of air, depth/width of soundstage. I just bought a pr of used Gallo Nucleus Solos this week & hooked up my system after being idle for a few years after selling ML-Sequel II's.
At this point I hope the reason is a worn tube & not a faulty tweeter or something!
Since you've owned your LS2 for a while you likely know what you are missing. I can say that mine can sound very analytical when combined with my SET amp and I would never use that combination, but with my push/pull it is magic. Swapping out speakers though will likely make a big difference in how your system sounds and the two speakers you mention are quite different in size and character. BTW I recommend GNS and Steve Huntley for their modifications but have absolutely no experience in purchasing tubes from Steve. That is not to say you should not, but I want to make that detail clear. He definitely knows the ARC products though having worked for them for many years. I buy my tubes from Jon up at The Tube Store in Canada. He certainly knows his tubes, as does Andy at Vintage, and Kevin at Upscale.
Appreciate your advice!
Will not be able to upgrade LS2 now (perhaps just tube), it's not the weak link by far. Amp is stock Ayre V3.
Desparately need to replace Pioneer PD-52 cd player, add power conditioning & room treatment as well as solving missing h/f problem 1st.
Just saw the Njoe Tjoeb 4000 in Nov Stereophile. Said it (output impedence 170 ohms) need to pair w/pre-amp with input impedence over 10k ohms. Is the LS2's over 10k ohms?