My experience with this preamp is vis a vis through my clients for whom I have re-tubed a couple of them so far.
A tube that will be hard to beat is the Brimar Triple-Mica Black Plate CV4024. Sometimes these will also be coded as 6060 or CV455, There are square getter and halo getter tubes and both are great. Probably $200-$250 matched pair. Holographic is the word everyone uses to describe tehes
Another tube that will be less warm and a bit more detailed//disciplined is the Valvo (Hamburg) Gold Pinned E81CC which is a premium (think 6201) version of the ECC81. Crystal clear, extended highs, controlled lows. Also $225-$250
A wild card is the Marconi B309. I kind of consider the Marconi B309 the much younger (and much cheaper) little brother of the GEC A2900, one of my all time favorite tubes. $400+
If you want the absolute best, and this is the rare time in tube rolling where there actually is a best, get yourself some GEC A2900s, sit down and start hearing your music again for the first time. $1k-$2k
The 7062 is another geeky tube to check out. Avoid the halo getters,. DFOIL FTW.