Studio Preamp retubing?

[eio]

I just picked up a used Studio Preamp that is in good shape although it looks (and sounds) to have the original or at least very old tubes in it so I am going to retube it.

Currently it has the following 5 tubes installed in it:

12AT7
SPAX7 12AX7 12AX7 12AX7

I primarily play heavy rock & metal.
Can some of you guys make a recommendation?

Also, what is the purpose of the 12AT7 & SPAX7 tubes?
I was just going to replace everything with Tung Sol 12AX7s until I realized that there were other tubes in it as well.

Thanks.