I'll assume you're interested in NOS tubes (and you should be if you're not!).
I don't know which 2 of the 4 matter most, so when I owned the 99 I swapped sets of 4 that I bought on eBay from reputable sellers.
In the realm of reasonably priced tubes, RCA black plates will tend to add some warmth and Sylvania black plates will add some detail and emphasize higher frequencies.
With one set of speakers I preferred the RCAs, with another the Sylvanias.
The 99 responds well to tube rolling and there are lots of options for 6SN7s. Good luck.
I don't know which 2 of the 4 matter most, so when I owned the 99 I swapped sets of 4 that I bought on eBay from reputable sellers.
In the realm of reasonably priced tubes, RCA black plates will tend to add some warmth and Sylvania black plates will add some detail and emphasize higher frequencies.
With one set of speakers I preferred the RCAs, with another the Sylvanias.
The 99 responds well to tube rolling and there are lots of options for 6SN7s. Good luck.