Linear operating range of an amplifier as a function of frequency
RF amplifiers operating from approximately 200MHz upward begin to obey stranger laws than more rudimentary op-amp circuits. Even below 200MHz, careful consideration should still be given to RF parameters. Normally the frequency response of an amplifier is determined by its transfer function, specifically any inductive / capacitive / resistive elements in its feed forward and feedback paths in addition to its unity gain bandwidth product. In terms of RF, while these general principles still apply, the end solution is not as straight forward.

RF frequency response becomes a combination of any band-limiting reactive components (L's and C's) in addition to the scattering parameters of the active amplifying device. These scattering parameters are very frequency specific and are normally matched for a specific frequency or band of operation. Characterisation of an amplifier outside its intended frequency of operation often results in degraded performance due to power being reflected internally due to an impedance mismatch. Of course this depends on the active device and whether it is broad or narrow band matched. The end result is a measurement called Return Loss, or more simply its analogue Voltage Standing Wave Ratio (VSWR)

Many active devices also specify an IP3 (3rd order intercept) product which allows a designer to estimate any harmonic content while in linear operation. The final transfer function will be heavily dependent on the RF match in addition to any other band limiting components, including stray artifacts such as trace inductance/capacitance. Through incorporation of any expected attenuation due to lumped/SAW/Cavity/strip-line filters, an accurate estimate of output distortion can be made.

More fundamentally, over-driving any amplifier will cause it to clip and introduce harmonic distortion.

So you need to be quite specific in how it is being tested and what is being observed. Generally, the significant majority of RF equipment is narrow band matched which allows for a closer to optimum match for noise figure, or power transfer. The narrower the match bandwidth, the more dynamic the frequency response will be over a broader test range primarily due to reflected input/output power.