boxsoft
First off, (before you buy anything) do yourself a favor and reach out to Brent Jessee at audiotubes.com, he is the most knowledgeable and best tube supplier out there. He can help with recommendations, and probably has experience supplying tubes for your equipment.
Great tubes are expensive. I can highly recommend the NOS Philips 5R4GYS, and the NOS Mullard GZ37....if they are compatible with your gear. The Philips is magical, and the GZ37 is really close
As others have mentioned, in most gear the 6SN7s have a greater impact on SQ. For NOS tubes (and I can recommend) the 1940s Sylvania 6SN7GT/VT-231 "Black Plates", and 1950s Sylvania VT-231 "Bad Boy" tubes......both of these will cost you $250 to $300 per pair.
For new production, my favorite is the Linlai E-6SN7s. They can be found for $155 per pair at jjyaudio, which is a very reputable Chinese supplier