Phono preamp tube rolling.

I’ve done a ton of tube Rolling over the years on the Ares Magnum. The 12ax7 slots make a huge difference. My 3 favorites, from most favorite to less favorite:
1. Mazda silver plates - These have a little extra energy on top, so maybe not the best fit for systems already leaning bright, but they far exceed the next 2 choices in all other respects. Almost as good as a component upgrade. 2. Telefunken - Ribbed plates if you want a little extra zing up top, smooth plates if you want warmer or smoother. Smooth plates are more neutral overall, ribbed can be a little more exciting. Both great tubes, and plentiful if willing to buy used, so don’t overpay for supposed NOS from a dealer.
3. RCA tall black plates. Good overall tube, close to neutral with a touch of warmth and sweetness.

The 12au7 slots, eh I never liked 12au7. You can sub 12ay7 or 12at7 in here. I’ve also subbed 12bh7, which is what I ended up suing the most - MUCH better sounding than typical 12au7’s. The best 12bh7 are 1950s Sylvania black oval plates with D or square getters. RCA of any kind are also good (warm) and Tung-Sol are decent (leaning a bit bright). The real problem with 12bh7 is their noise and microphonics in this application. But the Ares isn’t really the quietest phono stage anyways, so I just went through a bunch until I found a pair "quiet enough" and went with that. For a strict 12au7 sub, the GE 5814 gray plate triple mica were decent enough sounding (warm), easy to find quiet ones of, and affordable.

Rogue OK’d the 12bh7 sub for me, several years ago. These tubes draw 0.3mA more current on their heaters. I’m not sure if Rogue would say they OK it today (they seem to be getting more conservative in their recommendations over time), but I never had any problems.

In all 12ax7-based tube phono stages I’ve had (Hagerman Trumpet, VAC, Herron), those lead 12ax7 make a huge sonic difference. The follower tubes also make a notable difference, though not as much as the 12ax7 until you start trying type substitutes (e.g. 12au7 to 12bh7 / 12ay7 / 12at7) - then the difference can get almost as big.

@surfmuz
The 12 ax7/ ay7 / at7 aren’t really the same thing with different gain. They have very different plate resistances and drive capabilities, for starters. The Ares’ 12au7 slots are mu followers with a lot of flexibility - one element per tube used for gain and the other for output - so any of those tubes will work, but the 12ax7 is by far the worst choice here. It’s not a good output tube. That’s why I suggested at7 / ay7 / 12bh7.

I upgraded to a VAC Renaissance phono stage. Way more musical and organic, quieter, much better bass, and more detailed too. I used "way better" a lot because it’s all true - including the VAC’s sticker price! If I have one critique it’s that I actually like the CineMag blue-label SUT in the Ares Magnum more than the Lundahl LL1931 in the VAC (personal preference). But I bypass the VAC’s Lundahls with my choice of outboard SUT anyways - even sometimes with a CineMag Sky blue (great combo).

gakerty
Mullards have never lasted too long in a phono stage or any other application here, for the reason you colorfully cite :)

The Zestos looks lovely and its use of an asymmetric dual-triode is very interesting! Use of output transformers is cool, too. I'd love to hear one some day. 

Chelmers would be relabeled tubes from a large maker - could be USA or China. Probably USA if small signal. I had an old Hagerman Trumpet (wood tower type) that was supplied with Chelmer 6072 (12AY7) which looked like they were either old-stock Sylvania, RCA, or GE black plates.

It’s been a long time since I played with lots of vintage 6SN7, in a crazy headphone amp (and briefly later on in a Rogue 99 Magnum preamp). I do remember the Tung Sol BGRP sounded the most unique out of all, and the best to my ears. An extremely juicy, "technicolor" sound (like Koetsu) that was the only tube which seemed to fully deliver on the 6SN7’s purported promise as "king of tone". The Ken-Rad was my next favorite (especially BG variant), with bass 2nd only to the Tung-Sol. The Sylvania VT-231 seemed a little dry to me but was neutral and could be good in combination with Ken-Rad or Tung-Sol. I also liked the brown-base Sylvania 6SN7WGT, probably more than the tall bottle VT-231. I never liked the gray glass RCA VT-231 as much as these other makes, but it’s nonetheless a pretty good sounding tube. The Electro Harmonix 6SN7 sounded positively "solid-state" like by comparison.- a little grainy, and with a surplus of treble energy, but nice bass to balance it; honestly not too bad a tube for the money (and of course by now I’ve heard plenty of solid-state gear that is neither grainy nor bright). The Russian Tung-Sol 6SN7GTB sounds like a more refined version of the EH.

I also remember combining different makes seemed to stack only the positive attributes of each, rather than stacking or remixing the downsides (maybe this interesting quality is part of what makes 6SN7 so revered) - but if I’d ever had a full set of Tung-Sol BGRP, I probably would’ve just filled every slot with those. Problem is I kept getting used "nearly dead" ones that kept dying on me. It’s always been hard to find good ones and demand is so high people keep trying to offload duds.

For any of these old 1940s variants - GOOD LUCK finding a pair quiet enough to use in a phono stage or in the gain slot of a preamp. The old Sylvania 6SN7GTA/GTB 1950s "chrome domes" are easy enough to find quiet and in matched quantities, but aren’t as sweet sounding as the old tubes - they sound kinda like a slightly sweeter Russian Tung-Sol GTB in VAC 200iQ amps.

@lowrider57
I have a very good friend, younger and with better ears than mine - he says the exact same things about that Soviet 1578 tube. Yes the vintage/make is supposedly very specific and very important. Something abut round plate holes too, etc. I’d like to try them someday but understand they will be very hard to find (unless I can pry a pair off my friend), and my main system is not currently married to 6SN7 tubes.