Cary SLP98 Tube Question

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.

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.

Remember, you don't need NEW OLD STOCK. Just old tubes.

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.