Update on 6F8G and 6C8G.
Both models I bought in "round plate" versions.
The 6F8G while good, I wouldn't say blew me away. It's very likely the pair I got were really worn out.
It's impossible to find NOS 6sn7 or 6f8g round plates. Even used they're ridiculously priced $200-300, new $500+.
Imo not worth it at all, I'd say stay clear (purely because of price).
The 6C8G pair is just getting better and better with burn in.
At a fraction of the cost ($40 for the pair) and NOS instead of used, what a deal!
Right now they're competing for #1 spot, with my Psvane CV181-TII. More listening is needed, but the Psvane was $150 for the pair, so the price v.s. performance easily goes to 6C8G.
Also, here is my highly subjective resolution ranking for rectifiers:
JJ GZ34 - 0
JJ 5U4G - 3
RCA NOS 5v4g - 3
JAN-Sylvania 5Y3WGTA - 5
Mullard f31 fat base gz34 - 5
Preferred Series 274b - 5.5
Bendix 6106 (5Y3 Variant) - 6
Sophia Electric 274b mesh - 6
Psvane 274b treasure mkii - 6.5
Sylvania 5931 (5U4WG sub) - 7
GE NOS 5R4GYA - 7.5
Philco Sylvania 5U4G Hanging Filament - 8
RCA 5R4GY hanging filaments single bottom getter - 8
PHILIPS 5R4GYS (made in holland) - 9
NOS 596 (sub for 274A 5U4G 5R4)- 9.5
1940s RCA Jan Crc 5R4GY hanging filaments double bottom getter - 10
I only took into account detail retrieval (resolution). Each tubes has it's own tonal characters apart from that.
Did I mention it's also subjective
don't burn me at the stake, tube impressions are fickle.