Powers55...
If you have not used the phono preamp for many years the components in the RIAA equalization network may have changed so that the frequency response is as you describe.
Cranking up the volume control is no problem. All it means is that the phono preamp line level output is less than your CDP. When you crank up the volume control you really don't increase the gain of the circuits...you just attenuate their gain less. Probably better.
If you have not used the phono preamp for many years the components in the RIAA equalization network may have changed so that the frequency response is as you describe.
Cranking up the volume control is no problem. All it means is that the phono preamp line level output is less than your CDP. When you crank up the volume control you really don't increase the gain of the circuits...you just attenuate their gain less. Probably better.