Yashin, I've had the 5998 in the amp from the start, my 6080 was broken but I had three 5998s and was planning on running them anyway. I am a fan of the 5998 (the st version not the 5998a), every amp I've used them in sounded great. You get an increase in gain over the 6080, and I have noticed an increase in soundstage depth, control and punch. The base is tight with the Tung sols, not flabby at all. The Mullard Cv4003 is an excellent tube, it also has superb imaging. But to tell you the truth, after many listening tests I believe I prefer the CBS Hytron 5814a BP in the Crack. Ironbut recommended it on the first page of the tube rolling with Crack thread. I can't actually define what is better, it just sounds better to me. Plus as an added bonus, the CBS glows alot, unlike most 12AU7 variants, and I do like the glory of the glow!
Cheers,
Shawn