different power readings from different radios will change it very slightly (the SWR), if the antenna is grounded well the match should stay fairly the same with other radios of the basic same caliber . When you put your amp in line and check the SWR with it on high it should read slightly higher , no more then 1.5 and you should be OK. Lets say you have a 1.1 match and it goes up to 1.3 to 5 , I'd say you were OK , if it goes up past 2 , I'd say you had a problem , as I said , could be ground concerning the antenna ? or it could even be circuitry in the amplifier that has lost it's values in parts that help keep the SWR down in the first place. They have antenna analizers at shops that can be put on your system that help to ween out any problem you might have. IM sure others will have ideas for you on this. Good luck to you.