I can't answer your question directly but, remember you don't need to invest in NOS tubes to find out. You can buy many reasonably cost used tubes. Most input/driver tubes that Cary designs around are readily available. If that fixes the problem. You can begin to invest in NOS tubes if you really need (want) 'em.
FWIW, I was interested in this unit at one time and read a lot of user reports as well as reviews - it seems that your experience may not have been all that unusual. I noted a lot of comments about tube selection and the need for NOS (relatively expensive tubes) to get low noise and better tone than new production tubes. You might consider expanding your research and asking owners for specific tube recommendations. I recall some of the recomendations for for NOS RCA clear tops. BTW, I'm using some EH's in my Cary CAD 50II amp (with 6550 power tubes) and do not feel that they are overly bright.
Also (and I'm sure you are aware) you might compensate this brightness band by something as simple as speaker toe in, i.e. moving them so that your hearing thim a bit more off axis.
What is the rest of your system? I also have the Cary SLP-98 and do not hear the sibilance you speak of. I use Quad II Classic monoblocks, Ayre CX-7e CD player, and ProAc D15 or 1SC monitors. I've also used a restored Dyna 70 with this system and the Cary and again, no sibilance. For the heck of it, though, I did buy some NOS Sylvania Chrome Domes for the preamp, and the sound seems sweeter but not night and day different at least to me.
I had an SLP98 and I agree with you it can have that tendancy. You must have good speakers - many of them will mask sibilance. A friend of mine also had a Cary SLI80 and ended up having to get rid of it due to sibilance and a general projection of the upper mids that annoyed him (the two effects are linked in our opinion). He tried MANY tubes but it just never quite went away.
Also, I don't consider brightness to be the same as sibilance. Brightness is curable in many cases but sibiliance can be stubborn. Brightness is a higher frequency issue than sibilance is.
Check the filament voltage. Possibly it is not set correctly.
Thank you for the thoughtful replies -- I do appreciate them.
The rest of my system includes: B&W n802, Bryston 14bsst, and a Myryad mc100 cd player. My listening room is treated with a number of realtraps & rpg skylines. I have addressed the first reflection points as well.
I know that many feel the B&Ws to be bright, however to my ear the system leans toward warmth. They are definitely revealing speakers though.
I have played with the toe-in a bit and it helped some. I've also tried different interconnects with some success.
I hope I'm not giving the impression that I think the Cary is unlistenable. On well recorded material, it sounds quite wonderful. I had hoped that it would be more forgiving of poorly recorded cds.
I would like to try another source before I decide the fate of the Cary. Perhaps its just revealing the shortcomings of an aging cd player.
The Cary SLP 98L F-1 in my system sounds tremendous without a trace of sibilance whether using an Audio Aero CD 24/192 or a simple Music Hall CD 25 into MG 20R or Totem Arros. I have used Sovtek and China 6sN7's without a problem but got MAJOR improvement with NOS Sylvania 6SN7 GTB from the '60.
I found the stock tubes a bit brittle. Sylvania NOS was the cure. Very nice!! Note: You only need to replace the 2 front tubes with NOS.
Remember, you don't need NEW OLD STOCK. Just old tubes.
A good tube preamp will not tame brightness at all.Sibilence as was already mentioned is a different issue, but again sibilance is not remedied by use of tubes. In fact modern tube gear is as bright or brighter than many solid state pieces. The notion that tubes produce the sound you hear or heard from vintage equipment, which is frequently rolled off and syrupy, is a very widely held misconception.
That said the EH 6SN7 is not a great choice if you desire a fuller sound. Unfortunately there are no great sounding current production 6SN7s IMHO.
Sylvania GTBs are abundant, sound slightly euphonic, and do not cost much unless you buy from a dealer in these tubes. There are very reliable dealers but they do charge a premium, however the tubes will be what you are told they are, and frequently amongst the best around. In addition you can get true NOS. However, Sylvania GTBs are hardly your only choice.
It is only right that I warn you--- tube rolling can be habit forming. It's fun but can end up costing a lot.
You said your system in general, tends to be warm. You might try something intermediate. Try the less expesive types first, like the RCA GTB. These are also abundant and inexpensive, they are a somewhat less rich sounding and are a faster sounding tube. If you find nirvana with those you have managed to get away cheap. I do not recommend buying cheap tubes that have no name that you have heard about they are frequently true junk.
Beyond the GTBs things get a bit more difficult. You will need to watch the market be patient and see if you can get a good tube for a low price. It does happen, but you will never see a "holy Grail" go for a low price unless the seller is a known scammer etc.
Here is an example. If you want deeper bass extension and emphasis try to snag some Ken Rads,they are 1940-1950 tubes. Every version has essentially the same sonic signature. The price varies widely for arbitrary reasons, even though they are highly regarded by many enthusiasts. Look for metal deposits on the top of Ken Rad type tube, the getter is on the bottom in lightly used tubes the top is clear or has ver small deposits. Large deposits indicate that the tube has been well used (just to confuse you, they can still test strong but as a rule avoid significant metal vapor spotted tubes).
6SN7s tend to be microphonic etc. so there is always a risk of getting a bad tube. e-mail me on the side for more advice I have a fairly diverse and extensive collection.
I bought my Sylvania NOS chrome domes from Vacuum Tube Valley. Not cheap by any means, but the seller is reliable and knowledgeable and I felt comfortable buying from him.
Never had a sibilance problem (that wasn't on the recording) with my '98 (special edition w/oil caps & direct coupling)...but I never even unpacked the junky Chinese stock tubes. Get some good NOS 6SN7s, at least in the front 2 positions, and preferably all 4. The '98 is most definitely NOT the weak link in your system, just feed it properly.
I have a very similar setup...the F1 SLP98-P with Krell amps and N802 speakers.
A couple of months ago, I sprang for some RCA Clear Tops, also changed out my 12au7s and ax7s.
My CD player is the 303/300, so the synergy is there...but I have had excellent experience as well with my Denon 3910 for 2 channel SACD through the Cary pre-amp as well.
No sibilence...the sound is unreal. I will own this pre-amp until the day I die.
My listening is 50/50 digital/vinyl.
Check out tubeworld and give them a call...excellent advice for tube new-bees...I was a virgin when I picked up mine a year and a half ago.
The best dealer I have used in acquiring 90+ 6SN7s sells right here on Audiogon. I am not sure he ran an ad this week but he is "whoopdedooda". His real name is Len he runs an ad monthly on Audio Asylum under Len he is an expert 6SN7 dealer exclusively. As was mentioned there are several others sticking to home base try jsautter he has a variety of tubes. If you wish to use non audiogon sellers Tubeseller deals in 6SN7s only also an expert with links and information. Tubemonger has a variety of tubes but not much if any 6SN7s he has a good selection of accurately described pre amp tubes. I agree the Chrome Domes are very nice indeed. There are a couple of tubes however that given that name. Ebay is a minefield unless you know the seller or see something you know well. like a true "Bad Boy" in a mixed lot. Old tubes are in fact what you generally will find real NOS is not necessary, but a delight if you can find them. Len has some for sure he is meticulous in telling you the true nature of the tubes.
I have also dealt with Len for 6SN7s for my old SLP-98. Very reliable seller and excellent tubes. I have used Tubeseller as well with good results. Andy at Vintage Tube Services is also excellent.
Thanks for all the useful tube info! I'm going to try some older tubes in the slp98. Hopefully this will make some improvement.