Yes, especially the 6SN7s. The Sylvanis seem to be a great match for Cary gear. I'd do those first and see how you like it. There is definitely a difference between the stock tubes and NOS.
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A good place to start would be to find a NOS pair of Sylvania or Ken-Rad VT-231 bottom getters from the 40's. That's IF you enjoy a very accurate and transparent presentation. This is what to look for: (http://cgi.ebay.com/Pair-Sylvania-6SN7-GT-VT-231-Preamp-tubes_W0QQitemZ170271128352QQcmdZViewItem?hash=item170271128352&_trkparms=72%3A570%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14) (http://cgi.ebay.com/Ken-Rad-Vt-231-6sn7-1940s-Matched-Pair_W0QQitemZ220293427549QQcmdZViewItem?hash=item220293427549&_trkparms=72%3A1205%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14)The top getter Sylvania "Bad Boys" don't even compare(EXCEPT for the 6SN7W/JAN CHS, tall bottle). If you like a "warmer"(British- Brimar/Mullard) coloration: Try RCA, Raytheon or National Union VT-231's(again- bottom getters) from the same era. They all deliver great timbre, tone, frequency extention, wonderful soundstage and imaging(if the rest of the system/room is up to it. I've never purchased from those sellers on eBay, so- I can't vouch for them.
My intro to tube rolling began seven years ago when I bought a Blue Circle BC21 preamp.
I was pretty shocked by the transformation when I replaced the stock 6SN7s with WWII era VT-231s.
My favorites for the BC21 are the ones Rodman99999 suggests, the Sylvania and Ken-Rad VT-231s. Both tubes convey an enormous sense of scale while also imparting the positive qualities of this era of tubes. Refinement, dimensionality and a great midrange.
I feel the Sylvania is a bit more neutral, while the Ken-Rad seems more organic. I should say I have a preference for the black glass Ken-Rad over the clear glass version. In my set up, the black glass sounds, bigger, bolder and richer.
I am using an RCA VT231 (black glass) in my Dehavilland Ultraverve preamp and am finding it to be very microphonic. Has anyone else found this tube to be microphonic in this application, or is it possible that the tube itself is bad (the Sylvania Chrome top 6SN7 it replaced was not microphonic in this preamp).
I've experienced this a lot in my Blue Circle pre as well. More so with the WWII era tubes.
I'm not sure if it has to do with the age or condition of the tubes [even if they test well], or if it's just my preamp. It could just be that these "big tone" tubes are more susceptible to microphonics than other tubes.
I've never had this issue with any of the 6922 based components I've owned. DAC or preamp.
Gunbei- My Cary monoblocks take two pair of 6SN7's: one each as phase splitters, one each as drivers. When I used the VT-231 Sylvania(driver) and Ken-Rad(phase splitter) in combination, I was able to enjoy the strengths of both, and found no downside at all, as neither tube has an actual fault. I'm now using Tung-Sol JAN CTL roundplates(driver) and Sylvania 6SN7W/JAN CHS tall bottles(phase splitter). Talk about realism!