I can't comment on the 671 or R09 since I haven't owned them, but I owned and ran both the 702 and 722, and I think I prefer the A/D stage on the Sony D50 I have better than the SD boxes.
The 702/722 is obviously great for being bomb-proof and being a very good all in one choice, but if you've already got an nbox, I don't think the line-in/AD of the 7xx is necessarily the be all and end all.
I never did comps of the AD of the D50 vs the 7xx since I generally ran V3 digi> D50 and V3 analog> 7xx, so my comps are of the 7xx AD vs the V3 AD. Too bad, wish I had done that at the time...
Anyway, I'm now running PSP2>D50 line-in as one of my main rig configurations and like it quite a bit. If you don't think you're satisfied with the 671 run line-in, you might consider the Sony D50, or the new Sony M10 or the R09HR for that matter. Those will also make for a nice low-profile rig as well.