This is good information. I also picked up the same amplifier and the IRF530s were quick to self distruct.
When testing into a 50ohm dummy load I was initially only getting about 10 watts. The resistors that came with the unit were 100 ohm instead of what I found in the online schematic of 150ohm .
I made a low pass filter for it but I think the toroids are a bit small as they got really warm. I also made an attenuator pad at the input as I read that the pa does not seem to like more then about 2.5watts in. The pad was a 220, 22, 220 with about 4db calculated loss.
I found that with my o'scope the output waveform from the bitx was very distorted into the amp which could explain the low power and heating in my filter. In an attempt to figure out what was going on, I modeled the amplifier in ltspice .It did not work well as per the schematic I found. I had to increase the bias voltage and put a 10 ohm resistor across the two mosfet gates to get a nice input swr and a somewhat nice 70ish watt waveform out of it. This was simulated at 7.2Mhz with the ltspice model of the irf530 and all other components were ideal.
Since it looked good in simulation I attached the resistor and started increasing the bias voltage. I was not using a very small current limited power supply and while doing this there was a small puff of smoke and my supply voltage dropped to nothing. Oops...
I suspect not paying attention to the drain current may have been a problem. I think maybe they turned on and "shorted" at dc. Not sure if I trust the components that came with the kit though as they were in standard bubble wrap with no esd protection for the MOSFETs. They could also be fake MOSFETs.
I don't currently have replacements available but I might try your suggestion of using the 520s in parallel.
I appreciate your post,