Should I upgrade from Sylvania 6DJ8 to better tube

Each brand will 'tailor' the sound somewhat.
Amperex is a nice overall tube, but there are those made in Holland, USA, Japan,etc; they will all have their own characteristics...which makes tube rolling fun.
Some tubes will give a warmer sound, some leaner.

Don't forget the 7308 tubes, which usually provide better highs but not quite as strong bass compared to 6DJ8/6922 tubes.
If you search around you can still find good deals.

In my experience, there is substantial improvement over a 6DJ8 tube by going with a 6922 or 7308 which offer very tight triode sections.

Two top choices are 1960's Siemens 6922 and 1960's US made Amperex 7308 (either orange or white label). The Siemens is very clean, focused and slightly forward and dry, while the Amperex (especially the earlier white label) is holographic, airy, and presents a wonderful midrange. You could hardly go wrong with any of the vintage European tubes...

I am inveterate tube roller. The choice would have been abundantly clear a year ago. In a very short time prices for this family of tubes became insane.
I like the American Amperex nice neutral but musical, The CV2492/3 military Mullards a quiet tube unusually forward and uncolored if you can get the 2493. The 2 is more typically british and warmer. The regular mullard 6922 is very sweet but a little less detailed for my taste. The Siemens is in very broad terms a brighter sounding tube it really depends on which one you end up with there are many "Seimens" . They are all good. I have not had the luxury of hearing the Telefunken.
However as I said they are also all insanely priced or fakes now. I am deeply irritated by the astronomical price increases. Fortunately the tube market has a way of correcting itself so I am waiting before I buy- I need a quad.
The Jan Phillips 6922 is abundant, available and cheap because they were still being made into the 80s. That tube is a bit dry and hard but if your Sylvanias are getting noisy I would swap them out for them until prices cool off. Another alternative are the cheap russian tubes which some people really think are good .In particular the 6N1P I think? I don't care for them and really find the Sovteks et al. to be thin and screechy.
Sorry about being a downer but unless you have a very kind buddy you'll have to dish out serious green.

I agree with the above posters. I like any of the European vintage 6922's and 7308's. If your C-J uses 6DJ8's and isn't too hard on the tubes my favorite 6DJ8 type is the earlier Siemens CCa. Big, smooth, full with tight bass and huge soundstage. These tubes show up from time to time on Audiogon. Just make sure they are 1960's vintage or earlier. They were also made in the 70's but sound much more ordinary.

I'll stick up for the good ole 6DJ8: Amperex orange label from the 80s. Great tube that bettered several 6922s in my VK-3i. It has excellent dynamics and transparency without sounding shrill or brittle. There is more performance overlap between the different types than some would lead you to believe, in my experience anyway. The right choice depends more on your system and tastes than anything else.

Arthur