Stereo audio to 5.1 speaker fill with Dolby Digital Live over SPDIF?

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I recently upgraded from this HTIB to this receiver and these speakers.

With my previous LG receiver from the HTIB, using SPDIF from my computer to the receiver, I could set my Default Format on my PC to Dolby Digital Live (5.1 Surround) and with "Dolby Prologic IIx" also enabled, stereo music played from Winamp would speaker fill wonderfully to all speakers including the subwoofer.

On the current Sony receiver, when Dolby Digital Live is selected on my PC, stereo music only plays on the 5 speakers, but no subwoofer, and very quiet on the rear surround speakers.

On the same Sony receiver with my PC's default format set to any 2 channel setting (ie "2 channel, 16 bit, 48000 Hz (DVD Quality)") , I get sound beautifully through the front L and R speakers as well as the subwoofer, but not the center or rear surround speakers.

I'm new to all this A/V Receiver stuff, and can't seem to figure this one out on my own.
Can anyone explain to me why one receiver would fill to 5.1 just fine with DDL and PL IIx and the other not?
Anyone know of a way to fix this?

This is my gaming rig so I've been used to just leaving it on Dolby Digital Live to take advantage of positional audio in games, as well as speaker filling my 2 channel music, so I'm hoping to find a solution.

Thanks in advanced for any replies.

 

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I found the solution.

My receiver only does a low frequency pass-through to the subwoofer when the speakers are set to "small" in it's speaker config.

It also has a "Sound Field" setting called "Multi Stereo" that speaker fills 2 channel audio quite nicely.

So now I get great bass through the subwoofer and full speaker fill for any 2 channel audio, all with my PC set to Dolby Digital Live as the Default Format, so 5.1 content plays perfect as well.

Hope this helps someone in the future.