Rob - I'd bee concerned that if the ground weren't level or soggy that the speakers (even on a shelf) could settle....and possible tip or be prone to tip....I wouldn't be using sub poles at all just for that reason (top-heaviness)...
Fred had the right idea; rubber mats...and tripods for the tops....
Excellant point.... which means at a new location it would be wise to go ahead of time and scope out the setup area for sure..... in the case of the campground the area I'll be setting up on has been well packed as a parking lot for years.... and since I have been there before I am confident in setting up in the fashion I've described...
I hadn't thought of the sub pole issue since I don't have any yet so the thought never occured to me..... as my plan was to stack the tops right on the sub....

