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.
When you say feedback, what exactly does it sound like? Is it distortion or something different?

As far as the last part of your post concerning the volume level differences that makes sense. When you increase the gain on your phono preamp, you're basically turning the volume up, just by a fixed amount. It should be louder.
A very low steady tone
Sounds more like hum than feedback. If so and if it is beign generated in the preamp (maybe a ground loop) than it would have nothing to do with the phono pre.