1500 watt rms amp can be fine, just lower the gain a bit. those mtx can handle more power than they are rating at, as long as there is no distortion. let us know how they perform.
there's a link on this website for product reviews. in one of those reviews a magazine did a review on the mtx 7500 12" in which he put 800 rms into them and got only better performance. Those 8500s can handle the 750 rms with ease. I've even heard of people doing 1000rms a piece without any problems.
i had a mtx 7801 running to my 2 old 8500 12's and it worked just fine but i would make sure that you actually have enough availible power for the amp because i noticed that my amp wasnt running at max capacity since i only had 2ga wire instead of 0ga...but once i put 0ga wire to it there was a noticable difference in spl and sq...and you can run as many watts to a sub as you want just as long as there is no distortion because distortion is what damages subs!!!