Correct sound card was selected (Realtek High Definition Audio) & I've also tried Default Wave Device.
Correct playback device was selected from Control Panel > Sound > Playback.
Some additional information:
As well as the amp within the GRTT2 play deck I was using a Kaiser Baas Music Maker pre-amp. I notice that the green light on this device is no longer operating & I'm getting a window on my computer screen saying "USB Device Not Recognized".
When I remove Music Maker & plug the play deck directly into the line in socket at the back of my computer I'm able to record & hear the music simultaneously. However, I'm hearing an echo of the music & the quality of the playback, while without echo, is not nearly as good when Music Maker is part of the process.