Just for Grins and Giggles,
Take ANY simple SWR meter - put it in line between the Radio and the Amp - set to FWD, and CAL knob? Set it to mid-point - so you can see the needle go up half way...then keep a Wattmeter on the backside of (output of) the amp into that dummy load or antenna - and see about the "negative swing power drop" occurs as a RATIO of similarity between the two references.
Why? I'm trying to see if you may have a network problem, input level problem, jumper cable problem or battery supply issue...
You can glean some basic "run it this way" information and obtain an idea of how much power swing one pushes into the amp and the amp can deliver with - and or other power supply problems or if you have a bad jumper or two in this mess.
The network problem?
I've seen radios with poor soldering (cold blobby stuff) straight from the factory that gives you 4 watts and low SWR until you put it in a mobile situations and find the SWR will skyrocket and nothing seems to help - all because the radio on the bench had a bad part BEFORE the RF reached the radios own built in SWR meter. So a broken cold solder joint reheated, fixed it - but SWR would otherwise be ok into dummy loads and other antennas - until it's placed in a mobile setup where vibration took it's toll.
OR someone inadvertently inserted a cap in the wrong spot from the factory - and quality never caught it. I've seen caps "tack soldered" onto the rear antenna connector that really belonged elsewhere in the strip, do a similar effect to the radios own S/RF meter - due to bad placement.