Works fine here.
However, I am not sure what a mono 3.5mm plug adaptor is?
I am using a variety of soundcards which have standard coax outputs and the SP1 is very forgiving (consumer, pro, data, non data) in latching onto an incoming digitial signal.
I have the best interoperability with Windows system sounds, an ASIO media player, and TheaterTek using an old M-audio DIO24/96. The ESI-Pro Juli@ also works fine.
In digital mode I use pass through which will switch automatically to the PCM output rate 44.1/96 or to DD/DTS.
Given the current feature set of the SP1.7 my final setup is a bit more complicated with a dejitter box and a digital equalizer looped into the digital chain.
Cheers
Thomas