I cannot comment on the Rogue 99 specifically, but in other preamps my favorite 6SN7 tubes (most from the 1940s) are:
These come in three versions.
Two are tall glass bottle (one of those with a metal base)
and one short glass bottle.
Tung Sol Round Plate VT-231/CTL6SN7GT - They have black coated glass.
Ken Rad VT-231 - Some have black glass, some are clear glass. People will pay a lot more for the black glass ones, however there is really no difference. The black coating was an attempt to reduce RFI for some 1940's military applications. This is basically N/A today.
RCA VT-231/CTL6SN7GT. These have grey colored glass.
Did you buy the Rogue 99 used? I ask because the stock Philips 6SN7WGTA that came with the Magnum version are pretty good.
Yes I bought it used, I have had it for 2 years now and am very happy with it. I still have the Philips tubes just never tried them.
My Magnum came with Electro Harmonix tubes. Think it was 3-4 years old when I bought it. (store demo) Guess they switched. The EHs aren't that bad for new stock. Not as quiet (or detailed) as the Sylvania NOS in there now, but not bad. I know from speaking with Mark O'Brian at Rogue that he was never really happy with the EHs. Top notch company to buy from. Mark can put up with my constant questions! New NOS for the phono stage coming! Diana Krall is singing in my living room. Awesome.
The switch was probably because the supply of new Philips tubes dried up. They were military tubes from the 1980's.
Blue Circle was using them also in the BC21 preamp and BC2 mono blocks.
I got Electro Harmonix with my Rogue99 Magnum. I used them side by side with some Sylvania chrome domes Mark obrian sold me and they sounded fine. I'm running Crome domes side by side now and its great. Thinking about trying some Ken Rads or Raytheon VT-231's next. Anyone wanna comment on that?
I tried the Electro Harmonix ( bought from Upscale) in my 99 Magnum and they were micro phonic as hell. Nothing I tried would quiet them down. I bought new ones from Mark o Brien and they are fine. I honestly dont remember what brand they are though...
Pretty hard to beat either your Sylvania VT231's or the Kenrad's mentioned by sugarbrie. If you feel like experimenting a bit, you might want to try some 5692 tubes, which are in the 6SN7 family tree. I have used them with very good results, I personally believe that they produce a more cohesive overall musical experience than the VT231.
Sugarbrie mentioned the Sylvania 6SN7Ws: The two tall versions sound identical and are absolutely astounding in their musicality, sound stage and imaging. The shorter ones might singe your ear-hairs with their highs(unless your system is laid back in the upper registers). the TungSol 6SN7GT round plates(not with the mouse ears though)have very much the same attributes as the tall Sylvanias and work excellently together. I'm using one each in my Cary SLM-100's. If you switch out your Sylvania(bottom getter from the 40's) VT-231s for either of these: Your biggest improvement will be in depth of sound stage. I suppose you already know to shop for the lowest noise tubes possible, though finding either of these at all is getting harder by the minute. I've tried the Ken-Rad bottom getters and the VT-231s were a bit more natural, though not by much. I've not tried the CBS 5692 Red Base as it's scored consistantly low on every 6SN7 family tube shootout that I've read.
You only need 2 quality 6SN7's for the 99 Mag.
The 2 other tube positions have no effect on the sonics.
I cant remember which 2 positions to use quality tubes in. Ask Mark before you lay down big bucks for 4 tubes.
I think the right set of 6SN7s is the more important set. I spent quite a while trying to find the best combo a year or two ago, and to my recollection the pairing of left and right sets definitely made a difference. My efforts resulted in a pair of RCAs on the right and Tung Sols on the left (specs on the tubes are buried under the cover).
Sylvania BAD BOYS are good
TUNG SOLS MOUSE EARS are good
RCA Grey COATED GLASS are great
Mullards ECC33 = CV2821 are fantastic
Will be trying TUNG SOLS Black coated glass & round plates, KEN RAD VT231 and SYLVANIA 6SN7W black base soon.
Don't forget, tubes may not sound the same on differ amps, so, its always personal experience.
I agree with Maxmad. RCA Gray glass are great and don't cost an arm and a leg. Ken-Rads are very good also.
The RCA VT-231(grey glass/bottom getter) will lend a warmer(tubey) sound to the mids, the Ken-Rad VT-231(black glass/bottom getter) a cleaner presentation. Otherwise: they're about equal in detail, imaging and sound stage.
The combination of tubes that i have found most to my liking in the 99 is rca blackplates in the first position of each channel followed by a sylvania bad boy chrome dome in each of the second positions of each channel, this may not be to everyones liking but it sure works for me in my system!
My friend borrowed a pair of brimar 6sn7 Gty's from me and said they sounded great in his Rogue.
All the tubes mentioned are very nice. Which tube sounds best will depend on synergy with the rest of your components, not just your preamp.
I have all of the finer 6sn7s. Changing just one component most of the time,warrants me to change out the 6sn7s in my preamp for better synergy.
So,what works best in someone's 99 probably won't in someone others IMHO.
I'm using RCA VT-231 with two Sylvania Bad Boys in a headphone amp.. Terrific combination, but I have to say these tubes really benefit from Herbies Tube Dampers. In fact, two dampers down at the bottom of each tube is an entirely different ball game.