For 300Bs, in my opinion don't bother with anything but the best. I don't usually like to say expensive=best, but the WE and Takatsuki I've heard are just so amazing it'll bring tears to your eyes. Elrog are the others that'll be mentioned. They completely transform the amp into a real hi-fi beast. I've had mid-range, and they were good but the jump from stock tubes wasn't as big. Definitely buy once, cry once - and enjoy the warm, smooth and detailed sound of stock in the meantime.
Rectifiers for me are easy. Get some Japanese grey plates if you can, and enjoy. Stock are fine, and any other 22DE4 I haven't been able to find in 2 years of looking.
As for 6SN7 and it's equivalents, good luck! There's a wealth of information in this thread to tailor the sound to your system and preferences. I for one say look into quads w/adapters - 6J5/6C5, VR135/CV6/6C8C/7193, and more I'm probably forgetting. There is no best tube for these slots in my opinion, just flavours of awesome.
Enjoy the amp! It's a beaut.