Getting into comparing these things can get pretty complex, as you have clearly already discovered considering the units you are comparing.

The Arcam is probably closest to the Outlaw based on price and features (although the list price is still double that of the 990). Both units use the same chip for processing (the 49400), whereas the Anthem units use a Motorola solution (I'm guessing when you say Anthem you are referring to the AVM-30, although the Statement D1 is also an option if you are willing to venture up around the MC-4's price range). Both the Outlaw and the Arcam have similar input and output offerings and both use the same remote control (and in both cases I would tend to recommend a third-party universal remote like the Harmony or the MX-500 if you have a very sophisticated system). The Arcam offers a manual EQ, whereas the Outlaw offers somewhat more flexible bass management (four independently adjustable crossover settings) and an automatic channel calibration routine with an included microphone. The Arcam has HDMI switching instead of DVI, but it isn't clear to me if that switching includes any support for audio (a comment in the troubleshooting section of the manual makes me suspect it is video only). Based solely on a bit of poking around in the Arcam's manual, I would tend to say that the most compelling reasons to get it over the Outlaw would be aesthetics (or, along similar lines, equipment rack space) or sonics - and without heating the Arcam, I don't know how it might stack up to the Outlaw. The Outlaw sounds very good to my ear, so an extra $1000 or so would be hard for me to justify. Your mileage may vary. The Anthem AVM-30, meanwhile, is probably available for not much more than the Arcam, although the planned HDMI upgrade (which is supposed to include audio support) will likely cost several hundred dollars extra to add. It's been a very popular platform for many years now, so it's certainly a good one to have on your list of candidates.

I'll finish this off with a brief comment about the looks of the 990. While I'd like to be better able to read the processing mode that is currently active, I have been able to quickly identify which is which by general orientation from my viewing position (about 9' or 10' away), and I have found that the text display is extremely easy to read. The aesthetics are still a bit "minimalist" (a conscious choice on Outlaw's part, and one that people tend to either like or dislike), but when sitting in an equipment rack it seems to me to present a pretty respectable image. If it's not your style, though, it certainly will factor into your decision making process.