I recently sold my SLP-98L, but tried a number of tubes in it while I had it. One person you may want to speak with is Andy at Vintage Tube Services. He knows this preamp well and can make some nice tube suggestions. One suggestion he made to me is to use 6SN7GTA or GTB tubes in the buffer stage (rear sockets) and use 6SN7W, WGT, GT or VT-231 types in the gain stage. He said the buffer stage had a tendency to wear out the W, GT, and 231 types quickly. This advice worked well for me. Some of the combos I tried were:
Raytheon VT-231 (late '40's)/Sylvania 6SN7GTB (early '60s)
Raytheon VT-231/Tungsol 6SN7GTB (early '60s)
Sylvania 6SN7W (black base short and tall bottles from the late '40s)/Sylavania 6SN7GTB
Sylvania 6SN7WGT (early '50's)/Sylvania 6SN7GTB
Sylvania 6SN7WGT/Tungsol 6SN7GTB
Sylvania 6SN7GT ('50s Bad Boy tall bottles)/Sylvania 6SN7GTB
All of the combinations were interesting and it was a fun process. I would have to say the Raytheon was an excellent tube, right up there with the "W", but at a much lower price. I liked the combo with the Sylvania GTB better than the Tungsol GTB. An excellent combo, but a bit expensive is the Sylvania "W" (I preferred the short bottle as they were less microphonic) with the Sylvania GTB (the GT/GTB was nice too). The Sylvania GTBs I bought were from Tube World and Brendan has an excellent selection of cryoed tubes which is what he sold me. The Sylvania WGT is a nice tube and in this case I liked it better with the Tungsol than the other Sylvania GTB.
Here is a link I used to review 6SN7 type tubes and their characteristics:
One thing to be aware of is that 6SN7 tubes are getting rare and finding good, non-forged, low microphonic tubes are difficult and expensive. In my opinion the new production versions don't hold up, although Cary swears by the Chinese ones they use and maybe rightfully so as some designers are voicing their products to specific types of tubes by specific manufacturers.
Also, email me if you like, since I no longer own the SLP you might be able to convince me to sell a couple of pairs to you.