Stereo sounds better than surround for movies?

Your observations go directly opposite to mine over the years. I would guess two possibilities:
1. You are used to stereo and have internalized it as your reference.
2. Sub-optimal setup and/or room acoustics.

I would run the EQ in your Pioneer again and then manually check the settings. Do you have a sub?

I have the same experience with my hk avr2600. It looks like the signal is cumulated into 2 channels making it sound more powerful.

Sounds like you are not using surrounds. Did you set up your receiver so that it has these turned off? When you set it to stereo it would force all of the content into the front two speakers regardless of your settings but if you set it to surround and the receiver is expecting you to have surrounds you would lose all of that content since they aren't actually connected.

As to having more content go to your center in surround that's completely normal. The center is the most important speaker for movies outside of perhaps the subwoofer. Looks like you are mixing full size bookshelves with a thin flat panel center which could be why the center sounds like it has more treble as it could be a brighter speaker line and may not be capable of playing as full as your bookshelves due to the size. That's really the tradeoff of those thin speakers is they can't play as full of a sound. The room correction may be able to take care of this but you can also try and raise the crossover and see if the sub can fill in some of that mid bass. If none of that works to your satisfaction you may want to look into getting the matching center for your fronts.

Why would two channels sound more powerfull than 5.1?

He could be having lobing problems with a poorly designed 2-way center channel.

I agree with #1.... My father in law hates surround cuz he hates the center channel. He's used to his old Sony Quadraphonic stereo system, he likes all the sound coming out of his large floorstanders rather than the voices coming basically just from the center.

If your used to movies and music in 2 ch stereo moving to surround will be a difficult transition until you are fully used to it and realize that this is how it should sound.

You tell me. Lol

I just reported what i hear on my system.

I am talking about sound effects, not vocals. The center channel deals mainly with vocals.

My input is 2 channel pcm stereo and when playing direct mode, the sound is fuller. In 7.1 surround mode, the sound is spread out like a ring around you. By the way i have dual subs running in my left and right corners fed from the LFE output.

It accounts for what is there. Didn't you go through the setup procedure?

That may answer your question. If you are not telling it you have few than the full set of speakers/channels, it is likely that, with multichannel sources, it is throwing away the other channel information. With stereo sources, nothing is lost (but little is gained).