I think I just yesterday mentioned this, but yeah the arp is a bummer. I've talked to Sequential support a few times and they've confirmed that the rev2 needs to receive both clock and a MIDI start message from your DAW for anything clocked on the rev2 to play in sync (arp, sequencer, LFOs). For me, this means that the arp plays in sync if I start playback from the beginning of my Pro Tools timeline, but nowhere else (it's the only point at which Pro Tools sends a MIDI start message). And at least for me, it plays in sync even if it appears the the rev2's tempo readout takes a second to resolve the correct tempo.