I was looking into the Z560s and 641 by Altec. I like my "boom" in my music, movies and games and you do get lots of "boom" with the Z560s. RMS for them is 188W far more then the 641s. I heard both the 641 and Z560 (although the 641 were in a big store so the music quality is less then in a small bedroom for example) but although they were impressive I liked the Z560 better, much better. I have read a review (did my research b4 my purchase) on THG (I think) and what it said was that the 641 are better for music, leaning more towards classical, although the Z560s were excellent too - probably better for techno, hip-hop etc.
Spec wise the 641 are better, but that is only on paper. You will not be able to hear the difference that much. I had a very hard choice between those 2 but I have picked the Z560 because not only they were more pleasing to my eye, but also I liked the constuction of them better (more convincing the the 641), the weight of them (meaning biger magnets inside speakers = more richer, powerful shound) and the fact that they were THX approved.
When it comes to THX I agree with this comment 100%..........
"More specifically, in the multimedia speaker market, I can confidently say that there are other speakers out there that would meet the THX Multimedia requirments to become THX certified; however, many manufactures decide that the $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ required to get their speakers certified could be used elsewhere. Let it also be said that Klipsch doesn't get all their products THX certified ... just like Sony, Onkyo, Harman Kardon, et. al.
THX does guarantee that the product will perform at a "high level," but other products not carrying the certification may perform equal to or better than the certified product. Therefore, the argument that THX is nothing more than just a marketing strategy could be considered somewhat true."
.... but the THX also serves its purpose. Aside from the fact that it makes me happy to have the THX sticker on my speakers, I think that the THX certification gives you a piece of mind that the product that you are buying is good. This is useful for people who do not have the time or brains to do their research and rather pay the money for the convenience. I did my research, I knew that the THX is good but not Godly and that it might be a marketing tric to attract customers - I still bought it, why? because like I said b4, (call me stupid) but I just like to have the THX plastered on my speakers, that's all.
I hope that you come with the most suitable decision that fits you best. It all comes down to price, appearence, brand name, and looks (as the spec side is almost the same). Since you rulled out the Klipch speakers, my advice to you would be to listen to the other 2, and if you think that they both sound excellent (cuz they do) look at little things like speaker shape/design and pic the winner according to looks (after all we do the same thing with girls

). Good luck.