I admit that I've been busy and distracted this past week or so, and I think the quality of my posts are revealing of that. It just occurred to me that I spent a lot of time listening to the ML 15As in a dealer setting about a year ago or so, I almost purchased a pair, and cluelessly didn't put two and two together.
IMO, in your room as you described it, I would not recommend any subwoofers with the 15As. They not only don't need subs to get to 20Hz in-room for you (there are four 12" drivers per system),and they use Anthem correction software to tune the frequency response and phase. In my experience at the dealer it was difficult enough getting Anthem right just with the integrated subs, no less with two powered subs to place and tune on their own.
I recommend you get the 15As and see what you think before you go forward with subs. I really think you won't need them at all.
FWIW, I really liked the 15As. If the dealer had a better demo room it might have convinced me to trade in my Salon2s, but in the end I decided to stay put. I also wasn't sure I could live with the sweet spot effect that electrostatics have, and the upper midrange seemed a little bright to me, like I heard with B&W 800Ds. Nonetheless, after an hour of getting the set-up at the dealer right, I came away thinking the 15As are awesome speakers, and if I had anything less than the Salon2s he would have made the sale. And like I said, I think the poor venue was a big factor (as was the $25K + sales tax price).
Edit: and one more thing: to match the quality of the 15A's bass you're going to need a big high quality sub. Stuff like the SB-1000 or SB-2000 IMO need not apply.