A radio won't change the SWRs. SWR are set on the antenna and feed line if it is 1.6 on one radio on the same Frequency and 2.1 on another believe the lower reading. As power increases so do the SWRs. The Swing in a radio depending on how the Mod was preformed could be causing a Harmonic that is throwing off the SWR. This is easy to prove simply put a bone stock radio on the line and test it again. if the reading is lower then you know. If the harmonic is affecting the SWR you can bet that the radio won't sound good and is splattering as well.
PLEASE PLEASE explain how the amount of current ( wattage) into an antenna can change the impedance of said antenna?