Why does feedback go away when I do this?????

I have a Bryston BP25 preamp and 14BSST amp with Acoustech PH1-P phono preamp and high output Benz S class cartridge. When I set phono gain at 42 I have to turn up preamp past 1 o'clock or above to get volume. When I go past 4 oclock I get feedback but amp does not clip. I tried 62 gain and have no feedback problems. Is by turning volume to high level with low gain setting overdriving preamp??? Also at high setting I get a large difference in volume switching between source and tape. With lower gain it matches levels almost same. My Tandberg 3014A sounds amazingly close to source. Back to original question. I' wonder if preamp is over driven when knob can be turned so far. I thought by getting same manufacturer preamp amp this would not happen.