There seems to be a belief that the IRF5xx parts have no capability at VHF...
I have an amp using 8 of them at 32V that puts out a mere 210W at 50mhz. All of the parts are
IRF510s in a 4x4 push pull arrangement! The amps gain over all is about 13db.
I have also built the WA2eby push-pull HF amp and that design with 2w in gives me 55W at 40M
and 37W at 29mhz, that with a 28V supply. Also IRF510s so the part has capability though
generally if performs better at 24V.
THe bias on the IRF510 is often lower than optimum, Both amps (and other HF/6M projects)
run far more bias current that specified as it produced more power.
The problem is the amplifier has matching issues from simplified transformers. Additionally
at 30mhz the AC gain of the driver and predriver transistors (2n3904) is about 10 as compared
to an AC gain of 20 at 14mhz. So you seeing two problem one of driver that diminishes with
frequency and matching that is less than optimum with increasing frequency.
I've also built amps around the RD16HHF part and at 13V its good but it requires different
components around it to get the power out they can deliver. For a push pull pair with adequate
drive 20W is about it.
Before I'd go for changing parts I'd insure the driver and predriver are producing enough power.
It is also possible that the particular parts are on the low sid eof spec but the design is robust enough
to absorb that.
Another thing to insure is that your DC [pwer getting to the final is really 12-14V and not suffering
excessive voltage drop due to wiring or worse the power source fading at higher currents.
So dropping in RD16HHF1 in without changes will likely not improve matters much and it may
lead to instability. But if you have a pair you can try.
I'd expect more like 4W or more so something is not right.