And I'll second the endorsement on a headphone cue send! If you're doing a lot of live work where there's constant minute twiddling going on, having that as a safety to make sure you haven't screwed up whatever tunings your VCOs have, or whether the filters are responding properly, etc etc etc is worth the price of gold. Often I'll just jack into a parallel output with the phones, using a 3.5mm mono-to-stereo adapter, but sometimes even that isn't as helpful as I might like.