Solid advice. Over the past year I got a wild hair and bought multiple backups for every amp I use, either NOS stuff or tested/matched new sets from Tube Amp Doctor or TubeDepot. For what it's worth I've had good luck finding reasonable NOS RCA and GE rectifier tubes (5Y3 and 5U4) and 6V6 power tubes locally, but I'm in a large metro area. Good NOS 6L6 and EL34 tubes are crazy expensive though so I mostly have new tubes for those. I don't own any EL84 amps but my impression is that you might have to go new with those too. Also I highly recommend Soviet-era 6N2P tubes, which work as an 12AX7 substitute - you have to get a 9 pin "socket saver" adapter and carefully reroute a couple of pins to use them as the heater is 6.3V instead of 12V. The 6B2P-EV is the military grade version of that tube. I bought a box of 20 from some dude in Ukraine and whipped up some adapters for them, good stuff.