How can I still have the option to use phono imput

You're going to need a separate phono preamp between the turntable and the Krell preamp. In addition to boosting the cartridge output to match the line level input of the Krell, it will also apply the standard RIAA equalization to the phono signal.

If you still have your old preamp and it has a phono stage you can connect your turntable to it and go out of the tape outs into the Krell. You can use any input.