Reading the manual very carefully this time (i got my 505 since 2009) i found a feature very cool for me. In fact every track (part) is able to drive a midi channels; for example you can put part1 on midi ch off or 1...16 , and same thing for parts 2,3 and 4...many combinations are so possible: you can put some channels on off and just 2 of them on different (or same) channels, and so on. This is cool for trigger various synths and/or samplers/drum machines etc...the only "headache thing"
....i said that the 606 and the 505 are almost the same on midi field because i think the 606 has not the thing i saw yesterday on the 505; another thing curious to notice was the Program Change; i was trigger the mc909 with the 505 and i notice that with pc to "on" i could change patches on that machine and record that changement on the 505 sequencer...