Given those two choices, I'd upgrade the subwoofer first. You're right that a lot of people will tell you they love their SVS and HSU subs, and with good reason -- but there's nothing wrong with your Velodyne DPS 12 choice, it will be a fine upgrade to your Energy 8".
I know some people just love their surrounds, but to me -- a surround speaker gets such a light workout, and can sometimes be equalized to match your fronts/center with the receiver. I'd take a subwoofer upgrade over a surround upgrade almost any day.
Greg