i gather these are in parallel with the originals? can the driver circuit drive the extra 12nF of gate capacitance? (original gate capacitance of 14.4nF, total gate capacitance with 72 MOSFETS will be 26.92nF. also be aware this will likely require a change of the compensation cap in the voltage amplifier stage, or the amp could become unstable. also, in order to maintain the same clipping point with reference to input level, you also need to change the feedback resistor to change the voltage gain of the amp. i'm not saying 100% that those will be problems, but if you do have some problems with the amp after modification, you have some places to look for a solution...
actually, now that i think of it, you don't even have to take the transformer apart, you can add your extra windings on the outside, and put them in series with the original secondary. just make sure you are in the right ballpark with the wire gauge used on the secondary, and that they are wound in the right direction so they aren't out of phase with the secondary (you can determine this by feeding the primary with a signal generator and seeing if the added turns boost or buck the secondary). after adding the extra turns, and getting them phased correctly, wind a layer of kynar or mylar tape around them to protect them.