I owned a CJ 16LS for several years and am familiar with the 17LS. Both respond well to tube rolling, and yes, there are better tubes for these preamps than Sylvanias. I had the best results mixing tubes from different manufacturers - of course all w/in the same family. Blending allows you to draw on the strengths of different brands without, as it were, getting a fixed house sound.
Siemens makes a nice tube but I could never appreciate more than one per channel in CJ gear. More than one and the sound became a little too etched. I had very good results from a single Siemens 7308 with two Valvos. Valvo is a rebranded Amperex. Likewise the mix of one Telefunken with two Amperex 6DJ8 gave nice results. Generally I found the 6DJ8s to have a great upper midrange and airy highs - they tended to be a wee touch sweeter than the 7308s or 6922s. Mullards were a bit too warm and fuzzy for my ears.
The 7308 is a ruggedized version and should yield longer lifespan. 7308s have a tad better bottom-end and generally clear forthright mids with nice highs, but are not quite as sweet and warm. The differences can be subtle, but your 17LS is resolving enough for you to appreciate them.
In general, a safe bet is start with something like a set of four Amperex Orange Globes from the late 60's. After you're familiar with the Amperex sound, swap in one tube from another brand into each channel. The CJ composite triode design works best with triodes whose sections are more closely matched than not, but matching need not be obsessive.
Tube rolling is fun. I'd be sure I had a reliable dealer before starting through eBay - they are definitely out there, but there are also folks who sell used as NOS or counterfeits without even realizing it. Though a touch more expensive, reliable dealers such as Andy at VTS or Brendan at Tubeworld will deliver genuine tubes that sound like they are described.
Have fun with your CJ 17 - its a v. nice preamp!