Try Ssl7 first.
It has the most effect .
I am happy with Brimar
True NOS Gain Tested Brimar England CV1985 / 6SL7GT Vacuum Tube Brown Base | eBay
I also have ECC35 recommend by other folks. But it is in my storage. I may try it in 2 weeks to compare it with excellent Brimar.
I am also happy with Amperex Sn7
https://www.ebay.com/itm/PAIR-OF-6SN7GTB-AMPEREX-TUBES-NEW-6SN7-NOS-NIB-Vacuum-Tubes-Bugle-Boy/26392...
although I have Sylvania Bad Boys which I may try in a month or so.
Thomas