I have the Logitech Z623s, they are great. I'm not an audiophile or anything like that, but they they are loud, bassy (or not if you turn the sub down), and clear enough for me (read: average listener).
I don't know what the THX does, but they sound great, a lot better than my last speakers (although that could be because they cost 3 times as much as my last speakers). For what it's worth, I'd buy them again.