I don't know what converters they are using, but in the owners manual of the XR25/45 it lists the frequency response of the analog inputs including the 6 channel inputs at 10Hz-44kHz, +/- 3dB.
That would suggest some truncating of hi-rez signals, but my perception is that my Panasonic performs as well as my previous Yamaha and Pioneer receiver's analog pass-through for DVD-A & SACD. I know it's not an apples to apples comparison, but I don't feel like I've lost anything when playing hi-rez through the Panny compared to previous set-ups, FWIW.