I have essentially the same speakers, except that I have the MRX 512Ms for tops, rather than the MRX515. I think that both the 515 and the 512 take the same power, which I believe is program rated for up to 800 W (1600 peak). I use 2 XTi 4000s -- one for the subs (one sub per channel) and one for the tops (one 512 per channel). The 518 subs are 4 ohm cabinets, so they get 1200 W each from the Xti 4000. The 515 and 512 are 8 ohm cabs, so each one would get 650W from one side of an XTi 4000 if you're running stereo. If you're running a stereo mix, I'd recommend swapping the 2000 for another 4000 for the tops. If you're running the tops mono and bridging the 2000 to run both speakers, each one is seeing 1000W, which is more than enough.
Yes, we are going to run the system mono so sounds like the 2000 will work fine on the tops, thanks Jon