12au7 tube?

I'm also going to test different 6sn7's. I already have a pair of Sylvania 6SN7GTB Chrome Tops and I'm getting a pair of Melz 1578's (which likely will take quite some time to arrive since they're coming from Ukraine). They're kind of on two different sides of the spectrum where the SYL is a bit darker and with better bass and scary good midrange than the 1578 which is supposedly a bit brighter, super detailed and widest/deepest sound stage, but lighter in the bass.

I'll keep posting my findings as I get some time to do a proper run through them all (and all combinations...)

I never thought the Sylvania GTB was anything special. Very warm midrange and extended top. I liked the Raytheon 6SN7 WGT brown base which is a really nice sounding tube. For 12au7’s, I liked Raytheon’s, RCA black plates, Valvos, Brimars, didn’t like many Siemens because in my circuits they sounding bright. 

Yeah, GTA / GTB variants aren’t as good sounding as most of the older 6SN7GT tubes. You would need a GTA / GTB for 2 reasons:

1. Your tube slot (usually preamp or phono stage) is particularly sensitive to noise and microphonics. GTA and GTB tubes tend to be a LOT less microphonic than older GTs. Especially quiet are Sylvania chrome tops with the angled plates.
2. Your amp slot specifically calls for a GTA / GTB because it’s running a higher plate voltage and/or dissipation than spec’d for GT or 5692 (this one is even lower spec’d than GT so watch out). Sometimes driver slots in amps do this - like VACs, which specifically label these slots "GTB". You can often "get away" with running a GT or 5692 past its spec’d limits, but they might not last as long. Adding to the confusion: New Sensor makes an Electro Harmonix 6SN7 without "GTB" labeling, and a Tung-Sol variant that is labeled "GTB" - but they both have the SAME datasheet specs. Fortunately, it’s the higher GTB spec, so these are both safe for use in strictly GTA/GTB slots.

If you find you really need GTA / GTB (these are the same for audio applications), you might look for older 1950’s Sylvanias with deeper chrome tops and (sometimes) tall bottles. Sylvania really started cutting corners on production costs & quality in the 1960s. Their tubes also lost their beautiful midrange, sadly. Then they became Philips ECG and at that point, they practically sound like solid state devices.

@audiojan also try some RCA 5814’s with your Sylvania’s. They are warmish but seem to go nicely with the Sylvania’s I’ve tried in my preamp.

@inna rumor is PSvane makes many of the Ray’s tubes.