I like the look of the 2x6 top but I am a tad concerned about the boards curling up a bit and ruining my perfectly flat top.
What if I put a piece of 1/2 or 3/4 plywood under the 2x6's and glued/screwed them together? That should keep the 2x6's flat shouldn't it?
Or at least keep them from warping any more than they are when I buy them. I can run a sander over the top before staining to achieve a 'flat now' suface but I wonder if it will warp up the first day it doesn't have a full tank on top of it.
This concern about warping is probably just paranoia on my part but I like to cover all the bases before I cut once.
There are plenty of options. Gluing the 2x6s together at the edges is one way to go. You can use dowels/pegs to join them at the edges, or use pocket joinery, in conjunction with the glue. Probably the simplest thing would be to strap the 2x6s together from the underside with pieces of 1x3 strapping used for hanging drywall on ceilings.
I agree that you should have some kind of bracing on the back. Maybe not a full "X," but just some corner bracing cut from 2x4s.