So how do these compare to the Gec 6as7g (and tung sol 5998)?
It is very hard for me to compare these as generally, the 6AS7 has a very different sonic signature than a 6BL7. And what makes it even more difficult is GEC and Chatham did not manufacture the 6BL7, so we can't make that comparison. And further, I am most familiar with Sylvania 6BL7 and unfortunately, to the best of my knowledge, Sylvania did not manufacture the 6AS7.
However, there is a very well-written and informative review in Tweak-Fi of a six 6BL7 Glenn OTL that does a pretty good job of describing the essential sonic differences between the 6AS7 and 6BL7.
http://www.tweak-fi.com/apps/blog/show/42754587-building-a-reference-system-part-2-glenn-s-headphone-amplifier