That is a solid Graphite plate tube not the slotted ones I have come across a number of different Graphite shaped solid column tubes cross, round, semi oval. They are normally found branded Tungsol, Chatham or Bendix irrespective of the branding to my knowledge they were all manufactured by Bendix.
They are nice sounding tubes, I have had a couple of NOS ones in the past that have had a tendancy to give loud pops through the headphones during their warming up phase this effect dissapered after a numer of hours use but was a bit scary.
I have a number of Bendix 6080WB tubes, most are slotted ones, and never have the "loud pop" problem. The non-slotted tubes may take like 20 min to warm up. These are very nice power tubes.
The domino plate 5998 is definitly the better tube and I expect you will be able to pick up one for a similare price you just need to be paitent they do turn up. You would also be able to pick up a GEC or Mullard 6080 for less which are both very good options.
You might like to try the E80CC as a driver instead of a 12au7 a cheap Tungsram with the holes in the side will cost a lot less than a premium 12au7 I liked them immensly and prefered them to some highly rated and expensive 12au7 options. The E80cc are also long life tubes 10,000 plus the 12au7 life span is more often given seen around 2500hrs. They are a straight swap for the 12au7 and work well there is even a really inexpensive (couple of dollars) resister switch mod detailed on the forum which allows fine tuning the Speedball voltages for running the E80cc in the sweet spot.
Link for details and impressions of the mod
http://bottlehead.com/smf/index.php?topic=5989.0
