Well, it is important that you understand that I did all of these designs because I think each of them brings something new and exciting to the table. Its up to you to decide which you want, I cannot make that call. I like the SDX's smooth rolloff and fairly flat frequency response, if you want an amazing sub, this will deliver. However, I designed the dual Shiva sub to take advantage of the additional output of a pair of 12's, also, since each sub needs a lot less power to perform, even though the a single SDX handles quite a bit more more than a single Shiva, the pair should handle it easier. I need to point out that as of last weekend, I am now designing boxes for Kevin, so I just want to make Put that on the table. Kevin has made it clear that he doesn't want me to push people either way, just to provide information about the products and designs.
I haven't built that many subs, my first sub was a DB-500, I did 4 of those, slot ported. My second one was a pair of TC10's in a super small, high powered sealed box, JL audio would be jealous of that build, similar output to the F113 in a box the size of an F110. I built a pair of slot ported MJ-18's for music, those guys are very capable and I love how they perform in a wide variety of enclosures, its a very flexible driver. My my recent build was a pair of EHQS 12" subs that I put in a 4 cubic foot box, a friend of mine and I built that for another friend as a christmas present, I was very impressed with how that one turned out.
I have designed several subs because its just what I like to do, NeoDan and I have done quite a few designs for people on these forums and I feel that we fill a very real need for both the newbies and veterans alike.
About the amps, the XTI has very limited dsp, I doubt many people here will find it useful, that is not why I would buy the amp, I would buy because its a much more robust design than the EP2500, the behringer might deliver a tiny bit more actual power, but the XTI is a much more advanced design and will take more abuse. The Behringer will be louder, if you do a fan mod, it won't be able to handle the thermal power it does stock, this may or may not be a problem for you. It wasn't for me because I bought them to have lots of headroom, I rarely push them to clipping unless I try, but that was for mains duty, not sub.
Another amp option would be the Face audio amps that Kevin has on his site, they have a lower input voltage, making them perfect for receivers, other pro audio amps often times can't be driven to full output from the signal from a standard receiver, requiring a pre amp to be placed in line or diminished output. I know he likes to offer discounts, I would at least see what he is asking for dual shiva X and FS500 or FS700 amp together. The price on his site is not his normal asking price, I think its just the lowest price he's allowed to display. I can't recommend enough just emailing him about that package price, it always helps to at least know before you buy.