Behringer themselves have confirmed that the 2000w is peak output - and in tests it peaks at around 1800w so it's close.
However, they also do confirm if you ask them that RMS Constant output is in the 400-500w region as stated above.
The thing is, the headline peak figure is also a bit misleading, as in normal circumstances and playing, it's extremely unlikely anyone would ever exceed 900w peak.
But, as has also been stated, it's still remarkably cheap for a valve-pre 500w @ 4 ohms.