Hi "Rover TD5",
(Sorry, no personal.....no previous posts, no CT callsign....have you thought about joining?)
I wouldn't suggest you go inside your radio adjusting/setting/twiddling anything unless you know exactly what you're doing. (I know, that's why you're asking).
The "towers" with a "slotted screw" are almost certainly going to to be inductors/coils and you'll completely screw your radio if you start touching these without knowledge and test equipment. I wouldn't even start adjusting any of the potentiometers for exactly the same reason.
There is an old CB expression/phrasing/title called "Screwdriver Experts" for those that 'twiddled' with their radio's. It usually involved a 'jewellers screwdriver set' or a nice tiny screwdriver which others dubbed the "Golden Screwdriver". (Golden Screwdriver was also an expression for those that twiddled their radio, friends radio's, charged people for doing such and usually ruining every radio they touched!)
The Magnum 257 was/is a radio with good audio, the designers even provided an adjustment pot inside the microphone to adjust the microphones gain. Well thought out
If you've introduced a power microphone without much change then I can guess that your audio 'reports' are on FM mode?
It's quite common to get a quiet audio report on FM modes if one of you is slightly off frequency......If your radio is fresh from the factory, (despite modern Chinese junk claims), then you are almost certainly going to be "on frequency". Probably won't make the other station too happy but how old is their radio? When was it last serviced or calibrated?
Other stations to them might sound louder, but that could be the case of them also having older radio's that have been 'golden screwdrivered' to expand their FM deviation to the hilt.
My apologies if that doesn't seem to help you directly out, (I suppose a picture with "twiddle this" would be better?)
I hear too many reports of "your audio is quiet", "your radio is off frequency", "Chinese junk radio's", etc.
Fact is, if your radio is properly calibrated that "excellent audio but a little quiet" report is going to be the transmission that bursts through to a distant station and the over modulated transmissions will be lost in the noise...
But if all you're doing is chatting to mates a few miles down the road your "quiet" audio is going to annoy them. In which case you are looking for the FM Deviation
twiddle. Have a search for Magnum 257 mods.
Anyway, either I'm, "He don't know what he's chatting about", or I've been a little help. The little help bit was my intention
All the best,
(Oh, and think about joining. It's nice to be a Charlie Tango)