Start reading either "High-Power Amplifier Construction Manual" or "Amplifier Design Handbook"
You'll quickly realize that transistors are alot like relays. You input a voltage, and it allows current to flow through proportionally (sometimes). Swapping FETs won't help. Either beef up the power supply, bump up the rail voltages, or adjust the output current, by swapping/adding parts.
Others have hit it, though, when you mod an amp, you'll quickly find which part is the bottleneck... cause it'll melt or catch fire.