Also, I recently got my hands on a 5998 in good condition as well as a clear top RCA and a Gold Lion.
Some quick notes (using HD600):
The 5998 is more linear than the stock tube and with significantly better separation, although I think I still prefer the stock tube for some music for its fuller bass and low mids.
The Genalex Gold Lion is my favorite thing so far. Bass is tight and clean and hits hard, mids are rich and full, vocals are smooth and real, and soundstage is very good.
The RCA clear top has really pretty treble, but I felt that the combo of the RCA and 5998 was a little bass light. I have yet to experiment with the stock tube and the clear top but I suspect it'll be quite good.