We’ve had an old problem come up – namely, not being able to hear audio input from Reason despite having input monitor enabled.
The problem: you have made the proper connections between Reason and DP, so that you see audio output at the hardward interface in Reason, the proper Reason input is set to a DP stereo audio track, and the track is input monitor enabled, but you can not hear your audio through DP.
The test: record enable the track, along with the midi track that controls that synth in Reason. Play something on the MIDI keyboard. Did MIDI and audio data record? (You’ll know from the graphic waveform display if it did. Double click it to see it larger.)
If the answer is yes, the solution has to do with an audio input monitoring mode selection in DP.
In DP, choose Setup | Configure Audio System… | Input Monitoring Mode…
In the resulting dialog box, make sure that “Monitor record-enabled tracks through effects” is selected.
Rewire and instrument plugins are considered “effects” in DP. You can record their output, but not monitor them in “Direct hardware playthrough” mode. (Direct mode is useful for creating cue mixes in recording sessions without any computer latency.)