Kratos,
I've spent a fair amount of time with a GEC 6as7g straight brown base (driving HD800's) - and my view, at present, is that this tube is a bit over-hyped. No doubt they are nice tubes, with a unique internal structure, but 5998's and frankly many 6080's also sound great in my crack (with speedball and other easy mods) and can be had for a lot less money.
My GEC tube sounds like a very refined, premium, noise free 6080. In terms of comparing sonics, through my setup w/ hd800's, 5998's sound slightly brighter, livelier and more holographic. The GEC and 6080's are slightly more laid back and have a bit more mid-bass - particularly around 100hz where the hd800's have a significant spike in their impedance curve. This all makes sense given that 5998's/7236's lower your output impedance (considered a good thing by many) but also raise the gain, which may or may not be a good thing in your setup. (See also page 2 of http://www.pmillett.com/Wheatfield/rolling.pdf - similarly describing the sonic differences between these tubes.)
At approximately equal volume, if I throw on bass heavy tracks (e.g. 2pac's "u can't see me"), I perceive that the bass hits harder and fuller if I'm running my GEC 6as7g (or, for that matter, my other 6080's), when compared to any of my handful of 5998's. In the 6as7 thread on head-fi, folks with different amps and headphones have claimed just the opposite - that 5998's have more bass, which may be the case with the headphones they are using. As I understand it, the impedance curve of the headphone is going to impact the effect of the change in output impedance. Check out, for example, this thread on hd800's and high output impedance amps: http://www.changstar.com/index.php?topic=90.0.
All in all, I certainly would not drop the 2 bills ebay sellers are currently charging for GEC tubes. I paid $100 for mine, and I doubt I would be inclined to pay that much again. I like the tube, but I would say that I find myself reaching for my 5998's just as often. In fact, for the last couple of weeks, I've been experimenting with a number of dirt cheap JAN 6080's that have me second guessing whether it makes any sense at all to pay for "premium" 6as7 equivalents. I paid $15 for a pair of Chatham's on ebay that are dead quiet, not even remotely microphonic, and which had me listening to fournier's rendition of bach's cello concertos until the wee hours the night before last. If I manage to take my own good advice, I'll stop collecting surplus tubes and save my pennies for the bh dac.
My favorite a-socket tube is still a tung sol 12bh7.
