I believe I have a good understanding of what THX is and what it does. I happen to own a good THX ultra2 controller in the Onkyo TXSR875. I do not have THX speakers or sub but they are all of great quality JBL L series speakers and an SVS sub in the rear I use Infinity ES250's which are switchable bi/di/monopole speakers.

I just cant decide which mode I want to run my home theater in. I have used THX modes for a few months and then switched over to PLIIx on top of the standard audio of the dvd/blu ray. To my ear both sound really good. There is something really nice about the THX modes even with my non THX speakers. I always run my speakers crossed at 80hz. The non THX modes sound good with Audyessey MultiXT the sound is a bit more spacious maybe but also brighter.

THX modes provide better pans front to back of the room and a more airy larger sound effect.

I just cant decide :(

I just cant decide :(

Tough call. I don't have THX speakers or pre/pro. I use PLIIx and like it. Perhaps playing with your speaker location and the two modes will allow you to find a sweet spot. The next processor I buy will not have THX either and, although I would like the option, I can't say I'll miss it. ;)

One thing I do like about PLIIx is the way it works with dolby digital stations. I get a pretty good surround sound experience. However I've heard people prefer DTS, but I personally don't.