I still haven't decided for certain if I'll do this in the first place (I haven't really done much actual soldering of chips in the first place, plus I'll have to rig up a volume control somehow -- though I have an idea or two on that -- since most things don't actually let you adjust the volume of the digital outputs and the D3 itself lacks any sort of volume control, so the whole thing is definitely inconvenient) but obviously there's no point in even going to the troubles to try it if it's not really going to be any better. (As a pre-amp the TILMV358 seems to be fine, so while the AD8656 might technically still even be an upgrade for this purpose, it wouldn't be worth it for me at all if I wasn't going to be able to use it directly as a headphone amp.)
By any chance would anyone have any idea how the two really compare? I don't know if anyone would have had much opportunity to compare the two side-by-side, but perhaps someone might have enough of an idea of how each sounds to be able to more indirectly compare? It absolutely has to be really close to be worth all of the trouble for me. In fact, one person's "better" might actually be worse for me.
BTW, IF I am going to do this, I need a good source to actually get the AD8656. I got the AD8620 easily enough (already in an adapter even) from eBay, but the AD8656 looks to be tougher to track down. It seems that Mouser at least doesn't have it which is where I'd normally think to look for things like chips. It would be nice if I could get it from somewhere that wouldn't charge an arm and a leg for shipping, but regardless I'll need to find somewhere to actually get it. Does anyone know of a good source for OPAMPs like this?