Cary PH302 MK II Tube Question

'Head amp' in this case just means the amplification prior to the RIAA EQ ie the tubes closer to the back of the chassis. I suspect those tubes provide most of the phono gain - though I don't know for sure - so they may be the most critical. The overall gain of the MM phono is lowish in any case at 38dB. MC gain is provided by the inbuilt step-up transformers - though depending on the vintage of your unit it may not be suitable for lowish output MC's (say <.4mV).
My unit is the earlier build with 54dB total gain from the MC input, current units have a more sensible 60dB gain. Your manual should indicate what you have.

I have gotten superb performance with low output MC's using an external SUT. For something really special I highly recommend the Choir Audio H7. Like getting a new record collection!

I'm using NOS Sylvania Gold brand 6SL7's - they sound great! - but I haven't experimented with others. I bought my PH302 S/H and it came also came with the original Tungsol 6sL7's which were nowhere near as impressive as the Sylvania tubes.

You might also be interested in this thread regarding capacitor changes. Most highly recommended by me is substitution of the Audio 1 oil cap in the power supply - this will be a much bigger upgrade than any tube change IMO.

I read that thread but those changes would void my warranty so I would rather not do that at this point. I have found tube rolling to make a pretty big difference but I would like to understand better what role each set of tubes plays.

Agree tube rolling makes a significant difference. Though I know the Sylvania tubes sound great, I don't know how much use the original Tungsol tubes that came with my s/h unit had on them. So perhaps not a fair comparison.
I do wish the Tungsols were closer to Sylvania's, as the latter are considerably more expensive (and harder to find).

The tubes closest to the rear are definitely the input 'head amp' section.
Its easy to verify that that the tubes closest to the front are connected to the output coupling caps (see my other thread).

Enjoy your PH302, its a great sounding phono IMO.