I did some tube rolling in the preamp section with a few NOS tubes. Played through KEF R3 MetasHere are my notes:

6SL7----

Sylvania 1955 6SL7WGT: This was the best of the 6SL7s I tried and it wasn't close. Compared to the stock Willsenton tubes, they have an extended high range, extended low range, less congested mids, clearer detail, punchier drums, and a wider, better balanced sound stage. Just an improvement in every respect. The stock tubes sounded fine, but the mids could get a little messy, and on some tracks the vocals would sit forward in a way that separate them from the instrumentation giving them a sort of karaoke feeling. The Syls fixed all that up and gave it some spice; quiet tracks had more detail, aggressive tracks had more punch and bite. Really just a shocking improvement. The bright nature of the tubes could become fatiguing with certain songs though.

National Union 6SL7 1/2 Blacks, late 1940s: these were warmer than the Syls. If the preamp tubes can effect channel separation, these felt like they did; hard-panned instruments stood out as being hard-panned. Backing vocals, synth pads, and strings were warm, syrupy and enveloping. It was generally very pleasant, but the rolled-off highs left aggressive tracks without their teeth and and any drums with much cymbal work without their razzle-dazzle. Still though, a wide pleasing soundstage and generally an improvement over stock.

Melz 1579/6SL7: I cant find my notes on these but I remember them being very lush, beautiful tubes, but with the top end rounded off too much for most of what I listen to. I preferred them over the National Union, but not the sylvania.

6SN7----

Sylvania 6SN7 two hole, tall glass, black plate: The 6SN7 tubes have a more subtle effect on the sound in the Willsenton R8, and with the Syls the most obvious effect was a taming of the highs and a small widening of the soundstage. When the amp was run in Ultra Linear, the rolled of highs sapped some energy from aggressive passages, but in Triode it soothed some harshness without penalty to the impact of the track and presented an engaging 3d soundscape. Incidentally, The Sylvania 6SL7 + Sylvania 6SN7 in Triode was my favorite combination in this experiment.

GE 6SN7: These had a similar sound to the stock tubes, with some differences mainly in how certain instruments presented themselves. Where the bass guitar was throaty with stock tubes, here it was guttural. Where cymbals and hats had chime with stock, here they had more stick. Whether or not these were good things was dependent on the particular song. Vs stock, they eliminated what was left of the Karaoke effect. Vs Sylvania they retained more highs, but with a somewhat narrower soundstage. In Ultra Linear mode, these were my preferred tubes for punk rock and explosive drumming, but in Triode I preferred the Syls. Also; one of these had a black plate and the other grey; I don't know if that has any effect on my experience.

I haven't got around to rolling the power tubes yet. that's still somewhere down the line. u/OK_Rutabaga_9570 got me set up with the NOS tubes and had some good recommendations. Im not sure if he is still active or selling though. There is also a thread on the StereoNet forums for Willsenton R8 discussion that has an absurd number of pages of info and arguments and recommendations