Matching tubes, particularly in this circuit, is really not that important. But, if you must, the easiest way to do it is to use the circuit itself. Drop a tube in and measure the voltage on the cathode and the plate. Then, find another one with matching voltages -- you have a match.
For the earlier discussion about using 12AU7's, or any tube with a 12.6V heater, you simply need to bias the mosfet's gate differently. The gate needs to be ~4V or 5V higher in voltage than the voltage you want on the heater. In the original, we use an even divider off of B+ to put the gate at a little less than 24V, which in turn leaves the heater at about 19V. So, for the 12AU7, you want the gate to be at about 17V give or take. So, you just need to adjust the resistor divider.
Several additional things to think about --
1. Once the heater voltage is set, the heaters will draw whatever current they want. A 12AU7 will draw a reasonable current, but put a tube with a high heater current in the circuit and it can cause problems (namely heat and too high a draw on the power supply).
2. The mosfet is dropping an additional 5V which means an additional 3/4 a watt. Not a ton, but enough that you might need to beef up your heatsinks.
3. Adjusting the Rp and Rk (the resistors connected directly to the
Plate and
Kathode) would be prudent. To do this, you'll need to get out the
datasheet for your chosen tube and draw a load line. This is easy to do,
there are tons of web pages with basic instructions, and it will be a good learning experience.
-d