What's your amp? What do you want to achieve with the sound of the C2300? It'll take on the character of the tubes you choose, so these are questions to ask yourself. I owned it for a year with MC501 monos and spent big cash rolling a lot of tubes through the linestage. I didn't use the phono stage because I have a Steelhead II and listen to vinyl 99% of the time. Here's what I found in my system.
Telefunkens were too polite overall and a little thin on top for my taste. I bought three nice NOS pairs but never used them. Mid-'50s ribbed were a little punchier than smooth plates but not enough for me. YMMV. Early '60s Holland Amperex are tonally richer but a little fat and slow without enough clarity and extension on top. Medical Grade Holland Amperex (which are crazy expensive) from the same era, on the other hand, were incredibly focused and dynamic, like excellent 5687s but with better tone and bass. I liked them a lot despite the price tag. Mid-'50s RCA long black plates were excellent--3D, extended at both ends, really musical tubes, and less expensive than the others named above. The Med Amperex and RCAs, though quite different in character, were my favorites.
The only current production 12AX7s I liked in the C2300 were the Psavanes, which were very balanced across the spectrum. These are good tubes, IMO. Gold Lions had really good tone but seemed rolled a bit on top. The New Sensor Tung-Sols were just OK; Mullards too fat. I also rolled in all kinds of top 5687s but they were all too lean and bass-light for me in that preamp, which can be a little lean itself, even with MC501 amps, which are rich-sounding.
Now, all these results are simply in my system with my ears. Again, before anyone hammers me, YMMV. Teles, obviously, have a big following but they might or might not be your cup of tea. They weren't mine.
