What I did was change the function of the "Tape Head" input from a low level phono like input to a line level input. Biggest pain was figuring out a way around the oddball switching arrangement of the selector switch. Eventually figured out a way (which was to use the phono-tapehead equalization portion of the switch to do a preselect of phono or the new line level "tape head". The equalization RC was moved to the circuit board and hardwired into the now RIAA only phono preamp.
To avoid crosstalk from unselected inputs, I kept the "short 'em to ground" scheme. Also note I hard wired the preamp tube input to point 17, instead of making it go thru the switch like it used to. But I kept the switch connection on point 17, so the short to ground when unselected would still happen.
This is my same post as at http://www.audiokarma.org/forums/showthread.php?t=283202 in case you can't view the diagram here.