Just a quick comment on ProLogic II. I would disagree that you need a 6.1 or 7.1 set up to benefit from this. My kids love it when I pop in one of their pop/dance/house thumping CDs (2 channel) and I set the receiver on Pro Logic II or DTS Neo 6. Even though I have a 5.1 set up, the sound is a nice improvement over the 2 channel version.
So I would say that even with a 5.1 set up you will hear a nice, positive impact using Pro Logic II or DTS Neo 6.
Personally, I LOVE DTS. I have been using DTS for about 6-7 years, back when not many people knew what it was, so I am a little partial to the DTS Neo.
As for THX, it is supposed to be just a certification, like other have said, of a certain level of ability for a piece of equipment to meet some fiarly strict (depending on the level of THX - Select, Ultra, Ultra2) guidelines in that equipment's (receiver, speakers, and so on) ability to reproduce sound. With that said, you will find some receivers that have a THX Cinema (or something similar) DSP mode. I have not looked to see what benefit it has being a DSP mode, but if the hardware can handle THX, it should be able to reproduce a "fuller" sound spectrum than a non-THX piece of hardware. I think that the THX DSP mode just tried to compensate for sound nuances that are often lost in the recording, or in the average home set up. To me, the THX DSP is just one system's estimate of what you may want to hear. I like the more "true" modes like Dolby Digital, and DTS for movies, although like I mentioned above, I do use some others to create simulated sound for my center, rear, and sub channels.
Last note, you should get something that can handle 7.1 and DTS (Dolby Digital is always included, and DTS is there on any good quality receiver, and on some cheap junk). THX will be there if you start getting into the $500+ market for a receiver. Not all will have it at that price, but you will start to see it.
Since I was starting from scratch, I went with a THX Select receiver that could handle 7.1 and DTS. I figured if I was finally getting some nice speakers like Axioms, I should have a nice receiver too, and the THX certification just means a little piece of mind to me in sound quality.
Now, you could go all out and get THX certified speakers too, but why would you want to with great products like Axioms.
