Sounds like you have multitude of problems. It could be parasitics in the amplifier. It could be bad linearity, which could cause all sorts of intermod problems, which can show up as floating RF currents in your shield on your coax. If your coax is not very good you could be having simple RF feedback issues in the meter and the various radios.
Remember none of this gear is very well designed for RFI/EMI issues. The radio is not designed to be used in RF environments with strong field strengths (like when using amplifiers), so the design doesn't use devices like RF chokes in certain places making it possibly susceptible to RF feedback issues. One radio maybe different or better than the other in that respect. The meters react the same way, in most cases with CB gear. Even things like coax lengths may have a big affect on isseus like SWR readings depending on the current at the point where the meter is placed.
Grounding an antenna is key, especially with an end-fed like a mobile whip. If the antenna has nothing to electrically push against (counterpoise) by not mounting the antenna frimly to the grounding of the vehicle, it will use the shield of your coax instead. The shield will radiate to a greater or lesser degree (depending on its length) which can reak havoc on any meter reading. Not to mention high RF fields within the cab that cause mic squeals, radio overload issues and such.
Beleive it or not, the amp may be fine. The radios may be fine. It could simply be way you installed everything. There is a right way and a wrong way to do most things. Mounting a whip to a tool box may not be the best choice. Make sure everything has direct connection to the chassis ground of your vehicle. Not just by the black lead of the power cords. Run a wire from the gear-chassis, to the car-body/chassis. Make sure none of your cables are even multiple legths of 11 meters.
All that being said ... It does sound to me like the amplifier may be doing something wonky. Although, I do not agree with Shockwave as to the cause and certainly not "100%". In order for an amp to "self-oscillate" there needs to be some form of feedback loop within the amp itself (either directly wired, or induced or parasitic). This is why the term is self-oscillation. It could be more likely due to high input SWR between the radio and the amp. It could be simple swamping of the radio audio circuits, or RF output (or anywhere within the radio due to the high RF fields) or it could be floating common-mode currents coming back from an end-fed antenna not properly grounded (even though SWR may be low or "perfect").
You could try this as cheap experiment to see if the last thing I mentioned (common-mode currents) is causing the problem. Get a few of those clip-on ferrite thingees (techinical term) of the type that fit snugly around RG58. Clip a few (4 or 5) on your coax between your radio and amp. See what happens. If the problem still persists, try them next on the coax that's hanging under your tool box. Keep them close together side by side. Basically these will choke off some of the common-mode currents on the coax shields, which could be your problem or point of "feedback".
Remember none of this gear is very well designed for RFI/EMI issues. The radio is not designed to be used in RF environments with strong field strengths (like when using amplifiers), so the design doesn't use devices like RF chokes in certain places making it possibly susceptible to RF feedback issues. One radio maybe different or better than the other in that respect. The meters react the same way, in most cases with CB gear. Even things like coax lengths may have a big affect on isseus like SWR readings depending on the current at the point where the meter is placed.
Grounding an antenna is key, especially with an end-fed like a mobile whip. If the antenna has nothing to electrically push against (counterpoise) by not mounting the antenna frimly to the grounding of the vehicle, it will use the shield of your coax instead. The shield will radiate to a greater or lesser degree (depending on its length) which can reak havoc on any meter reading. Not to mention high RF fields within the cab that cause mic squeals, radio overload issues and such.
Beleive it or not, the amp may be fine. The radios may be fine. It could simply be way you installed everything. There is a right way and a wrong way to do most things. Mounting a whip to a tool box may not be the best choice. Make sure everything has direct connection to the chassis ground of your vehicle. Not just by the black lead of the power cords. Run a wire from the gear-chassis, to the car-body/chassis. Make sure none of your cables are even multiple legths of 11 meters.
All that being said ... It does sound to me like the amplifier may be doing something wonky. Although, I do not agree with Shockwave as to the cause and certainly not "100%". In order for an amp to "self-oscillate" there needs to be some form of feedback loop within the amp itself (either directly wired, or induced or parasitic). This is why the term is self-oscillation. It could be more likely due to high input SWR between the radio and the amp. It could be simple swamping of the radio audio circuits, or RF output (or anywhere within the radio due to the high RF fields) or it could be floating common-mode currents coming back from an end-fed antenna not properly grounded (even though SWR may be low or "perfect").
You could try this as cheap experiment to see if the last thing I mentioned (common-mode currents) is causing the problem. Get a few of those clip-on ferrite thingees (techinical term) of the type that fit snugly around RG58. Clip a few (4 or 5) on your coax between your radio and amp. See what happens. If the problem still persists, try them next on the coax that's hanging under your tool box. Keep them close together side by side. Basically these will choke off some of the common-mode currents on the coax shields, which could be your problem or point of "feedback".
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