I'd probably have to spend some more time digging through the Anthem's manual (which can be found
here if you are interested in doing a bit of research), but a quick scan brought a few items to mind. The AVM-20 and AVM-30 have THX (and the associated setting options). They also allow analog inputs to have some volume adjustment applied (to help compensate for volume difference between sources); digital inputs don't allow this. There are some automatic on/off timers (in case you wanted to use your SSP as a clock radio, although there may be some less silly applications for this feature). There are probably a few others hiding in there, and I was just looking at an AVM-20 manual that I had on the hard drive - the AVM-30 may have snuck some more goodies in. The AVM-20 has long been a very well-regarded piece of equipment, and the AVM-30 changed very little. On the flip side, the AVM-20 only has a single adjustable crossover point (unlike the four separate adjustments available with the 990); I'm not sure if the AVM-30 changed this.
Edit: The pre/pro comparison chart in my sig includes the AVM-20 and the 990 if you want a general feature summary.