I think the answer might be to simply get another set of plates, if the ones you have now won't let the planter turn down enough.

 As far as the mechanical drive goes, don't let that worry you.  I, too, bought a mechanical drive and I am oh so happy with it.  A buddy of mine bought the hyd. drive, and he has had hella trouble with it.  I don't think they're all that way, but his is having a problem in the wiring (they think).  The planter is on the ground, but it doesnt "think" it's on the ground; and therefore is causing a lot of problems..

 Unless you're changing your population a lot, I wouldn't suggest a hyd. driven planter anyway---but that's just my opinion.