If I remember correctly the Quad 33 has a 3-pin DIN connector for the phono input. Pin 1 is the left channel, Pin 3 is the right channel and Pin 2 is grounded (via small capacitor). So you can either make an adaptor cable (female line phono to DIN). Or replace the phono connectors from the Linn with a suitable DIN connector and connect both grounds to Pin 2 and left/right as above. Or add phono sockets on the Quad in parallel with the original DIN socket or replacing the DIN completely (depends upon how original you want to keep the Quad).
Hope this helps, regards, Richard