If you look at the disks at the top, those are the getters...you'll see the dimples - they are just basically "dotty",lol. I've only ever seen these on Mullards and Amperex. "Pinched waist" refers to tubes like this:
Hard to see in the pic, but you can tell that the glass curves in slightly. This is to make the tube less microphonic.
Spartan, glad you are liking your Woo! It's a fantastic amp, isn't it? What were you using previously? As for the preamp tube shutoff problem - does it happen to both tubes, or only one? If you swap sockets, does the problem move as well (if so, could be a bad tube)? I'd contact Jack and ask him about it - it could be a bad tube, or a bad solder joint, or a few other things.
As for different flavors of tubes, as a generality, you'll find that Amperex and Philips will tend towards more musicality and midrange warmth, Mullard/Brimar and other British tubes will have a warmer overall signature and tonality, and German tubes like Telefunken and Siemens to be more neutral, airy and detailed.
There are other options available, as those tend to be all higher demand tubes, but NOS Teslas from Czechoslovakia and Tungsram (not the new production ones!) from Hungary are good bets, and very affordable. The Tungsrams are copies of the Mullard tubes, and they have very high testing and production values, so they are worthy replacements. The Teslas are copies of Siemens ones, and a lot of them were made for military application, so again, tight tolerances and durability.
Also don't discount late production NOS tubes from the big names - although the vintage 50's-60's are more sought after, command high premiums, and are "generally" assumed to sound the best, the tubes made in the 70's are nothing to scoff at. They still sound fantastic, and as long as you buy them tested and matched, you've got nothing to worry about.
As for a decent and affordable power tube replacement, I'd defer to Skylab on that, or Jamato...the RCA grey plates are said to be great values, and I'd also look for 6080's - these are plentiful and very affordable. They are a tougher version of the 6AS7G, but don't have the coke bottle appearance (they look like your stock Woo tube). There are 6080, 6080WA, 6080WB, and 6080WC variants, all more or less the same. Not the greatest sounding tube (not BAD either), a little more bass oomph, but not as detailed as the stock tube, and lacking a little in overall dynamics, but when you can get them for $5-$15 each, you can afford to expirament and try some different flavors. You're going to get the most bang for your buck out of preamp tube choices though - get set there, then decide if you even NEED a different power tube...you may be perfectly happy with the stock one.
