Phil - I have/had similar problem with Kenwood TS-60S I used. It rolls
off on higher audio frequencies also. You can adjust the 4 tones for TX
fairly easily as described in the WSJT manual. In short, connect dummy
load and watt meter and see what you get on the A, B, C, D tx buttons.
Select the lowest one on output (usually the D/73 tone) and then play
with "FSK441 Amplitudes" in Setup Options (F2) for the other three
tones. The settings seem to be very nonlinear so it may not take much
reduction below 1.0 to get the levels down to match the lowest response
tone. I didn't have too much trouble with the receive curve on the
Kenwood. You might try IF Shift or something similar if the set has it
to see what effect that might have on Rx response.
GL - Bruce, N5SIX