Help with Schematic.

Mos transistors exhibit a purely capacitive input impedance at audio frequencies, and you can add more devices at the output in order to increase the intensity. But you have to increase the supply voltage too (carefully...)

The only problem is possible increase of distortion at the very high audio frequencies. You can cure this by increasing the current in the driver stage (which is low in my opinion) by decreasing the value of R15 (100, or 82 ohms) . The ordinary effect of this kind of tweak is better clarity, more velocity. It's important to install each pair of MJExxx on a little heat sink to avoid overheating, and some thermal distortion.

Regards, Pierre Lacombe