YAMAHA RX-V550 sound issue

Hi to this community.

I've bought recently this receiver and the Infinity TSS-750 Speaker System

http://www.hometheatermag.com/loudspeakers/204infinity/

I use a DVD-HDD recorder and a DIVX DVD player...both with Optical connection.

I have SAT and CABLE provider that only use stereo signal.
The receiver works great with DTS and DD, but is really a mistery the way it works with Surround Pro Logic sound. If you get close to the front left or right you can hear apart of too much white noise level, when the movie is playing voice or minor sound effects there is a strange sound effect very metallic and like a saw when you're shaking it. It makes the overall sound very poor... sometimes if you change to another source, like one specific movie, seems that sounds PL is working fine...

I've tried to use RCA connection instead of optical, but there is a screen mark PCM that dissapears and is almost imposiible to get real PL sound, many times you only hear only central speaker.

I cannot accept that only DVD or AC3 films are going to sound nice... My brother bought 6 years ago a Pro logic basic BOSE set with subwoofer built in decoder, amp and a external console with CD and tuner.... it really sounds great with Stereo signals over the 5 channels... the stereo sat provider movies sounds great when listened using those old speakers, working in Dolby Surround Pro Logic.... of course cannot decode DTS or DDigital signals.

I don't know what happens... I'm really confused, and I think that may be broken... please give me a hand to make me understand what can be happening.