I bought a pair of 13As earlier this year, but be warned if you have a large room with the speakers well away from the back wall - they won't sound good. No doubt the same would apply to the 15As. They need a wall behind them.I haven't listened to the 15a yet but did so with the 13a a couple weeks back. IMO it (13a) is 'the' sweet spot in the Logan line up. Without a doubt the new Masterpiece series are the best (hybrids) to date with respect seamless panel / woofer integration. I heard them @ Overture (Wilmington, De) so you know room acoustics were top notch. Mac 601's were driving them.
I bought these new speakers to replace 18 year old Avantgarde Unos, but the MLs failed to impress in my difficult room - the Unos sounded appreciably better (despite Anthem's efforts with the MLs), so I kept the Unos until I found a pair of Duos.
I'm selling my virtually new piano black 13As. Big disappointment, but I'm sure the 13As (and more so the 15As) will sound great in 95% of listening rooms.
You describe them as the "sweet spot" in the Masterpiece range. Cetainly the 15A is more overpriced than the others so I agree on a value-for-money basis, but not for ultimate sound quality. I'd have bought the 15As had I not been put off by the massive and seeminly unjustified price difference.
However many times you may hear these ML speakers at shows and showrooms and however many glowing reports you read - PLEASE TRY THEM IN YOUR OWN HOME before splashing out this sort of cash. They are not easy to resell. Mine are still for sale effectively unused with £7000 off the new price. Peter