Hello!
I am very interested for RB simulator MPIO HD300, but I noticed: at the Recording Settings -> Source there is no source 'Line-in', only FM radio and Microphone.
Just do not select the Line-In source at the current RockBox port for HD300.
But at the same time, this item (Line-In source) is at the Recording Settings->Source at the RB sim for MPIO HD200.
Best Regards.
PS sorry for my poor english