I got some 5998's coming in the mail, tube rolling has been an expensive venture - I think I'm finally coming to a close though.
Currently using a NOS Mullard CV4003/12AU7 (cryo treated, snake oil? probably) currently paired with a Sovtek "Winged C" 6AS7G which is a winning combo, but I've heard too many things about the 5998's to not give them a go, they were unfortunately the most expensive tubes I will hopefully ever purchase.
EDIT:
Got them in the mail, amazing tube, glad I got 2 - as I guess these will continue to grow in scarcity and continue to increase in price.
Question if anyone knows the answer, one of them actually has some writing on the inside of the tube "g F N", anyone know what it's about? never saw anything like it before: