When the S2400 is set as MIDI slave to another device, there doesn’t seem to be a way to set the Loop Recording ARM to ON, so that as soon as the START message is received, it begins recording on the selected loop track. As-is, you have to start the clock from the external device, then wait a second and then click record arm on the track pad – so you miss the downbeat on 1, and have to wait until the measures loop back around.
If possible, add the ability when stopped in Slave mode to click on a track pad again to toggle RECORDING ARM ON (red), so that as soon as it receives the MIDI Start message, it will begin the recording the loop buffer for that track. Would be useful for live looping set ups.