James,
If you like the 5U4GB (straight bottle), have you tried Raytheons.
When I was exploring that tube type, the short bottle RCA was my least favorite...a little slow, thick and dark in my system. A very tall bottle 50's RCA (with inner construction like the ST/coke bottles...no bottom mica) was one of my favorites, balanced, big and open, but depending on what it was with, sometimes "too good." They look like this:
http://www.ebay.com/itm/2-x-5u4gb-RCA-Rectifier-Tubes-Tall-Bottle-Black-Plates-3...And there is something interesting about the Raytheons I tried too...warm and textured but not veiled in my room. Like these:
http://www.ebay.com/itm/2-Raytheon-5U4GB-Rectifier-Tubes-5U4-Dual-D-Getters-Blac...The thing I found a little difficult with the GBs was their tendency to bigger/deeper bass than the STs, but I could work with it with other tubes. This is also a tendency of the MkIII though...big bass unless your room is very controlled or sucks it out.
All in all these days though, I tend come back to a particular bottom getter 50's RCA 5U4G-ST in my Torii MkIII, and in my CSP3, a GE labeled RCA ST.