jetson ...
As an engineer, the tendency is to analyze everything to death. As an audiophile, the goal is to duplicate a live music performance in the home in the best way you can.
Tubes: Not to worry about "used" tubes. They last a long time in a phono stage. What I would do would be to buy a new tube set as soon as I bought my used ARC PH-8. Not expensive at all.
On the cartridge ... I recommended the ART-9 because I've been through many MC cartridges including the AT OC9 MK III. The ART-9 is the much better cartridge ... as it should be for twice the money. It has a slightly higher output at .05 as opposed to the .04 output of the OC-9. This makes the ART-9 an almost perfect match for the PH-8. The ART-9, like all AT cartridges, is an over-achiever, competing with cartridges in the 3k to 5k range, for under 1k.
Hope this helps ...
Frank