Yeah, that Mullard that Doc supplied with the "Demo" Crack/Speedball for CanJam was real nice! IIRC, it was mated with an RCA 6080 (or GE?)
I certainly don't have the level of experience that Doc has with 12au7 types but I tried a few of the common subs. As usual, what tube you're using in the 6080 position and any component mods makes a difference in the 'tuning'.
Subs I've tried;
Tungsram E80cc
CBS/Hytron 5814a
Brimar 13D5
RCA 5963 bp d getter
and a few other 12au7's that just happened to laying around that really weren't memorable.
6080 types
Sylvania 6080
RCA 6080 (supplied w/kit)
Tung-sol 6080 graphite plate
Tung-sol/Heintz and Kaufman 5998
My fav's depended on whether the Crack had the Speedball upgrade or not.
Stock, I like the Tungsram E80cc with the Sylvania or the graphite plate Tung-sol. Pretty subtle differences so your cans and ears will make all the difference.
BTW, I didn't have a 5998 before I upgraded so that could be total hogwash if you have this tube.
After the upgrade, I really like the CBS/Hytron 5814a with the Tung-sol 5998. I think the CBS is in the same ballpark as the Mullard I mentioned above so I'd say that Doc's recommendations with the 5998 are all bound to be over the center field wall as well.
Some particulars;
The Tungsram E80cc is slightly higher gain and very clear/transparent. For many of the 6080's,it was too much of a good thing IMHO. Very impressive sounding and great for demo type listening but not something that I'd enjoy day in and day out. IOW audiophile sounding.
I really found the Sylvania 6080 to be something of a sleeper. It went well with a large majority of 12au7 types including the E80cc. Great bass, fluid mids and highs and just didn't do anything wrong.
The TS graphite plate had a quicker sound and with something like the CBS/Mullard it was a great match. With others, it seemed a little bass shy.
The Tung-sol 5998 with the CBS, Mullard is heaven IMHO. Bear in mind that I have made a couple of mods to my Speedballed Crack such as film output caps, slightly upgraded pot and silver signal wire but those pale in comparison with the improvements that the Speedball and the 5998 made. They were done before the Speedball since it would be tougher to get in there after the extra boards were installed.
As usual, YMMV. These listening impressions were done with my AD2000.