Well Mark, how about the "4S MK IV Universal". Just remember, you asked for it.
In a future build (if Matt is not sick of charting my course) I may do a build which can switch from 12AU7 to 12AX7.
How about we do one better. A "Universal" 4S preamp for the dedicated "Tube Roller". This design, with no changes to the circuit whatsoever, will handle the 12AU7, 12AY7, 12AT7, 12AX7, and 12AZ7. Here is the design and calculated performance numbers.
The full volume stage gains run from -9.9 to -28.1 (19.9dBv to 29.0dBv) with fairly low distortion numbers. (Remember that an audio taper pot at half volume is a 20dBv loss and these gains are a lot more reasonable looking, i.e -0.1dBv to 9dBv at half volume.) All these distortion numbers are quoted with a 1v peak to peak input voltage. The 12AX7 has the least input headroom but it also has a bucket load of gain so that shouldn't be a problem. I had to go to a 250kΩ volume pot to control distortion, but even with that pot, the output impedance numbers don't look too bad. So long as the amp (or whatever) you're driving has an input impedance greater than about 100kΩ the loses should be minimal (worst with the 12AX7; about -2.2dBv at about 8/10 on the volume scale).
I may build one of these with a 12AU7 cathode follower output stage just to drive prototyped power stages under test. So, let me know what you think.