Re: Dimarzio 7 string p/up choices
Well, I would generally say to look no further than the Blaze neck to start with. I have one in an RG7620 and it's probably the best neck pickup I own. I wonder if it might be a bit too high-output for your needs though. Certainly, it sounds stronger to my ears than a Duncan Jazz. Still, the 5-way switch in your 7621 should allow you to have the neck pickup wired in parallel in position 2, for which the Blaze sounds absolutely sweet.
The only Dimarzio 7 string bridge pickup I've used is the X2N-7, which is clearly not for you. Based on what you want, a PAF-7 might work, or possibly an Air Norton 7. But I can't speak from experience on those, I'm afraid. Your choices appear to be fairly limited as their 7 string pickups tend to be high-output affairs.