Hi guys!
Great to follow your conversation!
I have talked to Steve about 5751 tubes for my ZP3 a few weeks ago. He said that the specs are o.k., so these tubes are an alternative!
I use in the first and second slot a NOS Tung Sol 5751 (grey plate) with really good results (well balanced, holographic, not too bright). They produce a much more involving music than the Teles 12AX7 (I tried ribbed and smooth plates)
In the third slot, I use a Telefunken smooth plate 12AU7. (good tube, but after your experiences I will try a RCA Triple Mica 5814A)
My rectifier tube is a Brimar 5Z4GY (very detailed)
I am really happy with my combo! I never thought that tubes in a pre phono stage could change the soul of music so deeply.

All the best
Leo