John explained it well BUT PLEASE READ ON

For MIDI your are OK using the Audio Box to drive the Ketron SD2

BUT you should NOT try to use the AUDIO Box to bring Ketron SD2, or ANY line level audio, back to PC - its NOT BUILT for line level signals!

http://support.presonus.com/hc/en-us/articles/210044713-Can-I-connect-a-line-level-source-to-my-AudioBox-USB-or-22VSL-


and from Page 3 of manual

"Power User Tip
: Active instruments are those that have an internal preamp or a line-level output.
Active instruments should be plugged into a line input rather than into an instrument input.
Plugging a line-level source into the instrument inputs on the front of the AudioBox USB not only
risks damage to these inputs but also results in a very loud and often distorted audio signal."


sorry for that bad news - hope you have another way to get line level audio into PC

Win10Pro,i9,64GB,2TBSSD+20TBHDDs,1080TI,BIAB'24,Scarlett18i8,Montage7,Fusion 8HD,QS8,Integra7,XV5080,QSR,SC-8850,SPLAT,FL21&others,Komp.14,IK suite&others, just a guitar player-AXE FX III &FM9T, FishmanTP, MIDIGuitar2, GK2/3'sw/GI20