Hi, I'm a bit of a late comer on this thread....but I have a few observations. If jesseday2 does not need more gain, why not just use a cathode follower? A 12AT7 will have a gain of about 0.95 and a 12AU7 about 0.90. With the stepped control it would seem that is what is needed. I really hate to cut signals down to boost them back up as in a 10 to 1 input reduction feeding a gain stage of about 10. That increases the noise level at the output. I hate noise.
Hi Mark, 1/2 watt ones ought to be fine. Yes I would use the same tube for both first stages and the same tube for both followers. You could do the other way, but as you noted you can't do tube rolling if you do that. While all the various tubes with 12A?7 bases will work in the circuit, without doing some math I am not sure all will be in the best operational range. Funny thing is that 12AU7s and 12AX7s can often use the same cathode resistors and to a certain extent the anode ones but the 12AT7s bias differently and are unlikely to be in the most linear region of operation with those values. For small signals it may not matter much, but when you get past about a volt or so it can lead to distortion. I use a program to check out possible values and then run them on the distortion analyzer to verify them. PM me if you want regarding the process.
Edit: I see from the chart that the 12AT7 is IMO not a good choice. Anytime a preamp stage exceeds 1-1.5 % distortion I figure that a different approach is needed. Even then that is about 5 times what I like to see in a preamp stage.
Good listening
Bruce