I think what you need is a cambered allmountain board with rockered nose. It handles like a regular board but has extra float for the deep stuff. They are perfect for what you have said you want to do.
Boards in this category are:
Tapered boards like the burton malolo, barracuda, capita charlie slasher are hybrids but they are boards that I would rather have on a powder day than on a groomer day. All are fine in the backcountry (but I find the charlie slasher to be 1) too wide underfoot and 2)not a fan of the flat camber under the back foot).
Full-on powder boards like those mentioned already (fish, gyrator, etc) are not so well-performing on groomers. Especially the gyrator. Not recommended for all-conditions riding. The Hovercraft is similar to the fish but is quite stiff. This takes away some of the fun factor, but lets you get your carve on, and is more versatile for backcountry riding.
Another idea is the Burton Sherlock. It's a hybrid rocker (rocker with camber under the feet) with side effects tech on the nose and tail which provide extra float. It's basically a twin built for pow. I find it carves well and the other weekend in nipple deep pow it floated fine, with a centred stance. I think it will perform a bit better than your gnu, but I guess it's prob too close, unless you sized up. It rides short btw.
So anyways, flagship (check for defects first), Experience (awesome board, better than the flagship) or Berzerker are my recommendations.