You can try different lengths of coax jumper cable.
It should be significantly less than 6 feet, or significantly more than 7 feet.
11ft, 4 inches of RG58 (with hard, clear center insulation) will often fix the swing on the amplifier.
You can coil up the jumper to keep it out of the way.
The high / low switch is a trick to switches out the impedance balance in the feed input of the amplifier.
This causes the watt meter to show slightly higher power, but causing the impedance to change greatly when modulated.
Switching the high / low switch to Low, switches in the designed feed balance of the amplifier.
In the old days, the amplifiers had this balance circuit, but didn't have a switch on it and it was designed by the transistor makers as part of the amplifier.
So if there is a high / low power switch, place it in Low and try some different length coax jumper, between the radio and the amplifier.
As the radio sees an impedance change while modulated, the radio protection circuit will reduce the power output of the radio, so it is important to keep the drive impedance balanced.