Strange, I got most of mine for a couple bucks, the most was $4 each and the cheapest, for free.
Friend of mine down in central FLA just came across a large "stash" of NOS tubes, asked if they were worth anything. I told him to send me a listing, to see if they are all plinkers or if there is any "gold" in there.
The use of 6AV5GAs is no secret, they are all over the other audio foums, and some vintage Bogen amps use these in PP operation, that may be their "desirability".
12AV5GAs are even cheaper, and just as good. Same for 25AV5GAs, etc.
6FW5s are a heftier version with a 18W PD rating. The compactron 6GE5 and 6GT5 are also near equivalents to 6FW5.
I suspect that the 6GE5s, 6GT5s, 6GV5s, etc, are also very good in triode mode, just have to try them out sometime, in triode strapped mode. These all have 12V and 17V filament versions.
I started the "next generation" 6AV5GA amp today, chassis drilling and punching is complete, and mounted all the iron, sockets, etc, just have to wire it up under the chassis. It all goes onto and into a Hammond 12X10X2 black powder-coated steel chassis. Transcendar3K, 10W OPTs, Hammond horizontal mount PT and a hefty PSU choke. I got a good hand & wrist workout with the nibbling tool
I bought myself a cheapo drill press at "Tractor Supply" a couple weeks ago, and it definitely makes the metal mangling a LOT easier and quicker.
/ed B