Digital Performer with Unisyn (a.k.a. DP) is tremendous for working with external synths. You can solo individual MIDI tracks and record them back into DP or let them run live into an input in your software. This will all depend on your audio interface. I would suggest a larger multichannel audio interface for that if you don't already have one. The MOTU interfaces are rock solid for DP or Logic.
Logic and Pro Tools can do this as well, but I think DP is most strongly suited for sequencing.
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