Unless Onkyo has changed their ways, I personally would never buy one of their surround receivers. The reason has to do with the fact that with Onkyo there is only DPL II and DPL IIx, but no plain DPL. Why does this matter? Be patient, this will take a bit of explaining:
"Matrixing" is a troublesome word, in that it is used to describe two different processes. The kind of matrixing that is done with Dolby Pro Logic (and the center rear channel in Dolby Digital EX and the matrixed version of dts-ES) is where they take, in the studio, more channels than the finished product can contain, and mix them down in a special way to fit on those fewer channels. To make life simpler, let us confine our discussion for the moment to Dolby Pro Logic (hereafter referred to as DPL).
With DPL, they have 4 separate channels in the studio. The front right, front left, front center, and rear (also called "surround", which is why it is "S" in the quote below). These four channels are then mixed together down to two channels in a special way:
From:
http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/208_Dolby_Surround_Pro_Logic_Decoder.pdf
Your DPL decoder at home reverses this process to give you 4 channel sound from a two channel source. This whole scheme was developed for theaters to use, so that there would only need to be two channels of sound on the film. There was not room on the film to easily add more channels, and also, this way, they could use existing projectors with two channel readers, which then feed the signal to a special decoder. But it also is perfect for two channel VCRs and 2 channel sound on analog TV.
Now, of course, they can do sound differently than when DPL was invented, and they can keep the channels totally separate from each other. Dolby Digital and dts (in their 5.1 versions) do just that.
Basically, the matrixed Dolby Digital EX and the matrixed dts-ES mix the rear center channel in the right and left rear in a manner similar to how the front center channel is mixed into the front right and left in DPL.
Now, finally, we can get to the other idea of matrixed sound, and that is where you make up channels that never existed in the original recording studio. This is what happens, for example, when you apply DPL IIx to an ordinary two channel CD. The recording studio did not have a mix for 7.1 sound; they had a two channel mix. The processing that is done at home in this case moves sound that was intended for the front right and left speakers to other places. Hence, it re-directs, or mis-directs, the sound to other places. Now, whether this creation of previously non-existing channels is a good thing or a bad one is a matter of preference. But it simply is not what was originally mixed, whereas the result of using DPL on a DPL encoded movie soundtrack is not creating any new channels that did not previously exist, but is only recreating what was in the mixing studio before it was forced onto only two channels.
So, if one uses DPL II or DPL IIx on a soundtrack that was originally encoded as DPL, one is re-directing, or mis-directing, sound to where it originally was not intended to be. Whether you like the result or not is what should determine whether you do this or not. But do not imagine that you are simply decoding the sound; you are processing it in a way that was not intended when it was originally recorded. It is like using "Hall" or "Studio" or some other DSP mode to process the sound in a way that is, hopefully, pleasant.
With Onkyo (unless they have changed their ways), you can only use DPL II or DPL IIx. So Onkyo receivers cannot properly decode DPL encoded sources. Most other brands give you a choice and allow you to decode DPL sources with DPL, or to process it with DPL II or DPL IIx, if one prefers.
