Okay with that info..
Output/power tubes, try to find a 5998 in America for a decent price (it’ll take time). It will add some nice energy/quickness to all driver tubes you pair with it. Another option is a
GEC 6080, here’s a reputable UK seller (I’d get that over a Mullard). Chatham 6AS7G is a solid third choice.
Driver tubes, this is more difficult. I’ll avoid the rare or expensive options. I am tying to keep in mind what I prefer with my HD6XX but I mainly use those for jazz and that kind of thing since I purchased my Beyer’s.. This is where you’ll start collecting, as some tubes (add adapters) work better with certain genres. I’m going to miss many as there are just too many to list..
RCA 6SN7GTB - (see photo below) mid 50s, short bottle, side getter, copper support rods, offset plates. very nice forward vocals. Will do folk and rock nicely for you. Can do hip-hop ok, won’t have the impact or presence as the Sylvania 6J5GTs below. It lacks some low end and detail. This is a bargain tube that will walk all over most new stock tubes. Mine is labeled “Dumont” I paid about $5 for it.
RCA VT-231 - (6sn7) - an okay entry level choice, will lack some treble and clarity but it’s smooth and has a strong mid-range. Can be dependent on the output tube though. Vocals are also not it’s strong point so I hesitated in mentioning it. Don’t pay more than $40.
Foton (6sn7) 1950s only with ribbed gray plates - Soviet tubes that can be a great bargain. Will be harder to acquire with the war. I purchased mine from Ukraine. Dynamic sound for the cost. Less midrange and more treble and vocal clarity for rock but will be quite dependent with the output tube.
Sylvania 6J5GTs (gray plates only!) you’ll need 2x with an adapter - that signature “house” sound, mid dominant, vocals are nice and a step above the RCA above, not super detailed but just enjoyable, works with electronic and hip hop well. Can usually be found for $10 a tube!
Ken-Rad VT-65 (6c5 metal cans) - my go to for jazz. Less moving than the Sylvania above with hip hop. Less treble, vocals are forward but instruments have a spacious sound, even on HD6XX which don’t have much of a soundstage. These truly have a unique vintage sound to them. It’s a bit strange and hard to explain. Also can be found for less than $10 a tube.
7N7 tall bottle with oval micas only (more rare) - chrome dome down to half of tube, you’ll need an adapter.
Tung Sol VT-94A (6j5g) - more rare
English tubes.. much more $$$ but try a Brimar 6SN7GT (lovely mids) and a Fivre 6SN7GT. Then pretty much any genuine English 6J5Gs/L63. Mullard, GEC, etc.
That’s just a few options.