Viking MegaQuilter, Pfaff Grandquilter or Janome 1600P (all 3 are made by Janome, just different covers) have all the features you stated except dual feed & low bobbin. I have the V. MegaQuilter and I love it, it's great for piecing and machine quilting. Juki & Brother make similar large harp machines.
Pfaff is the only line with dual feed on all models but most of them are well over $1000 and just regular arm size.
The 6600P is a great, heavy duty machine ... gives you the most for your $$. To get equiv. features, including the wider harp, you'd have to spend $2000+ for a Pfaff Expression 4.0. You really need to shop and try these models out to see what you like. :)