One thing to keep in mind about the RFX-75 is that it replaces the radios driver and final. That means the ERF-2030 is only driven by the radios pre-driver at a level usually less than 300 milliwatts. That means the output of the ERF-2030 used in the RFX-75 is significantly less than the 20 watts it might make as the final in a radio driven by over 1200 milliwatts from the driver stage in most cases. The fact that driving the amp harder will not produce more than the 400 watts suggests those transistors are already driven into saturation and well beyond the point of gain compression.
I'm not a fan of using any RF device in this type of package for applications above 50 watts to 30 Mhz. Motorola, one of the most trusted names in RF transistors NEVER manufactured a part over 40 watts in these cheaper 3 pin packages. The 40 watt versions were also made with the metal tab as the emitter or grounded pin to reduce inductance. This is very important when dealing with higher levels of RF current (required at 12 volts) and the upper limits of the HF spectrum. Power, efficiency and stability all suffer when this is ignored because the transistor struggles to maintain a path to RF ground as the power goes up.
The unfortunate truth is they perform worse than the lowest rated Motorola MRF455 RF transistors made 30 years ago in these amplifiers. That's why you need at least twice as many to come close to the output of the old bipolar transistors. Any more current through a single stage and the circuit falls apart or pops the transistors. Incidentally, if one of your transistors were burnt out, the 100 ohm resistors across the output combiners would be smoking when you keyed. Seeing 400 watts output may just mean you have an accurate watt meter.
