Why not try both? Connect an HDMI cable from the Oppo to the Onkyo, and also connect multichannel analog audio connections, and switch between them to find out which works best with your particular combination of components.
Just connect to the 876 everything via HDMI and be done with it.
Using analog cables will be pricey and to gain sifgnificant gains there you'll have to step up substaintially which means still more $$$ before you may realize gains using the Oppo's decoder.
Change the pc on the Oppo if anything. That's a bigger help as I've found out on my 980 The Onkyo too likes better pc's. Do those and stick with HDMI as ICs... and do that Audyssey setup as described in my other thread via links supplied by Kal for it's proper setup.
When, if, you compare both type of connection, spend enough time with each to see, hear, the sound quality. Not just the frequency response ( base, mid, treble)but also all the details, the space, etc. If you try quick switching you'll end up with the same conclusion as double blind test: everything sound the same.
Blindjim says that cables will be pricey. It will if you don't already have them, which is the case for most hifi maniac. On the contrary, not being satisfied , that's really pricey.
With the Onkyo 800 series, you can hook up a source two ways at once. one via HDMI and as well that same one via the multi ch input thru SE cables. Six or eight of them depending. That's the pricey part. 3 or 4 at least need be identical. All should be, to fairly test this issue out properly.
Following those connections of course one can after some playing via each IC type, more readily A/B the two interfaces.
I used only a stereo pair of ICs + HDMI off my Oppo, initially and listened in stereo only to see the diffs if any between the two interfaces. Noting little I moved up my cable inventory, AA SE III, MIT S2, HT Magic II, Nirvana SX Ltd, SR Resolution ref x2.
Until I got into the HT's & up, I could detect little gains if any. Some by way of tonality were noticeable. Sweeter, smoother, or more defined, and again, a bit more than marginally perhaps, though I was after all, unable to justify the addition of 3 more pairs of $800 @ set, or better, simply for my movie viewing interests.
I repeated this exp on the proc to amp facet too, hoping to gain something perhaps by adding or examining the diffs from my MITs & Audio Art cables. that took more time and no direct A/B'ing. Just notes,.
Again. Similar results. though none IMO were cost effective there either. Even if you or I could find 3 to 5 MIT S2s at 1.5M ea. at a good price of say $325 pr. half that for a single... yeah, good luck there. Ya got over 8 bills into it then.
Maybe with a better processor & source & amps things would prove out better, that ain't the case here or the OPs Q. So IMO on this level or with good outboard amps, good cables are all that's needed for film.... not uberexpensive or great two ch ones.
Look elsewhere to get more from the system... such as optimizing the room, sub, power cords, speakers, their placements if possible, etc.
HDMI is just hard to beat sonically or financially on the levels being discussed here.
Which HDMI CABLE MAKER? I've only tried 3 or 4 brands as source, and use the big Blue Jeans for the long run of 26ft. to the projector. Again, the diffs are so minimal as to not represent to me an area for concern.
use HDMI and buy more movies with the left over $$$. As the last poster said "it's primarily in your mind" as you'll not be missing anything substantial... likely not anything noteable for that matter either.
I'll hang onto my analog cables, thank you. My tube gear has more resolution and detail ability than does the receiver's preamp & proc... and neither my tube preamp nor mono block amps allow for HDMI.
When higher level makers of preamps & amps include HDMI interfaces, the high end buyers will have been done a great service and cable choosing and paying for as well, will be just so much agregate folk lore.
But then, who wants a tube setup with onboard digital encoding or decoding of an analog signal anyways?
it do work well for HT though. That's for sure.