[Amps] solid-state amp "suckout"

[Scott Townley]

I have a broadband, solid-state amp that seems to have a large-signal "suckout".  The small-signal gain across the HF band is as expected, 10.5dB decreasing to 9.5dB at 30MHz.  But under full drive, the output at 17m is down more than 3dB, and at 15m down about 2dB.  Input VSWR is <1.5:1 for all bands, small and large signal.  This is just the amplifier modules; no bandpass filtering is in place.
Any ideas or suggestions?  
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Scott Townley NX7U
Gilbert, AZ  DM43di