I'm hoping for a 500 HD. That amount of power seems just right to me-- enough headroom to provide really nice low end, but not so much power that you blow things up if you twist a knob one millimeter in the wrong direction.
The main thing I'd like, though, is the pre/post EQ control for the XLR out, but on something less than 800 watts.
In the meantime, I'm using a Creation Audio Grizzly Bass on my GK 1001RBII as a backup to my Rumble v3 500 head. The 1001RBII does have enough power that you could blow things up if you were to twist a knob one millimeter in the wrong direction... but it also has the pre/post EQ switch AND a mute switch. And the Grizzly Bass warms it up something close to the Rumble. Not exactly as plush and luxurious, but similar. (The Grizzly does the same thing for my 400RB, but without quite as much beautiful low end.)