To my untrained ear, I would say there are ranges where both outperform the other when volume matched by ear without a meter, but the F15HP is usually more apparent throughout the range AND runs at a lower gain / volume than the S1500.
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I’ve got my 2’s crossed at 90hz and it sounds seemless with my JL e112. No localization at all.
For around $100 you can get a Umik-1 and if you can get a computer or laptop in the room, pair it up with REW (Room EQ Wizard) which is free. Using these tools and a bit of reading, you'll be taking frequency response and waterfall graphs of your room in no time. Ears are always a great test, but I find that running test tone sweeps and seeing EXACTLY how the different crossover frequency settings integrate the S2s with the subs is what you want.
Had mine at 60hz for a long time until I eventually did this. Turns out 90hz with Dynamic EQ Off and MultEQ XT32 Flat was the magical setting for the flattest freq response.
Cheers!