Below is the press release from their web site.
It seems like they've just redesigned the circuitry from the ground up and added
a few features. Main added feature I can see for
the autocom is the de-esser.
The price difference isn't very big so it's
probably best to go for the pro-xl as they've
probably ironed out some of the bugs etc...
Personally I'm looking for a gate/low-end compressor and I'm thinking of getting a MDX2600,
as it also has some valve sim gizmo.
"The PRO-XL Series is a completely re-engineered series of BEHRINGER dynamics processors, in which proven and reliable circuitry is coupled with clever innovation. And we have pushed the performance benchmark to a new limit. Existing features, that have made us a standard in the pro audio industry and among professional musicians, have been re-designed from the ground up and are now complemented by a myriad of new features.
The MDX1600, MDX2600 and MDX4600 now all feature our revolutionary IKA, IGC, IRC and IDE circuitry plus a switchable low-contour filter that prevents pumping.
Quality does not take second place to fancy new technology. Ultra low-noise 4580 operational amps and state-of-the-art THATŪ VCAs for more headroom and crystal-clear signal path as well as high-quality ALPSŪ potentiometers and servo-balanced inputs and outputs with 1/4" TRS and gold-plated XLR connectors complement the new design.
VAD (Voice-Adaptive De-esser)
The voice-adaptive de-esser removes "S" from vocals and sibilance from overly bright signal sources
IDE (Interactive Dynamic Enhancer)
The interactive IDE enhancer restores high frequencies lost during heavy compression
ATS (Authentic Tube Simulation)
Tube simulation for extra warmth and transparency of classic tube circuitry
IKA characteristic of the compressor section
The program-adaptive IKA compression combines the advantages of "hard knee" and "soft knee" characteristics
IRC characteristic of the expander section
The interactive non-linear IRC curve that is best suited to human hearing for virtually inaudible noise suppression
IGC characteristic of the limiter section
The two-stage IGC limiter provides reliable and inaudible protection against signal peaks"
(for more info there's a lot of spec/user manual pdfs at behringer.com)