(I think it's the other way round) C1/L1 fill in the dip at 1.6KHz - see Mr Marley's link to the measurements earlier in this thread.
C2 and R2 are part of the 2nd order filter on the bass/mid.
Some more pointers in the review:
"crossover
In view of the monitor requirements, the crossover is elaborately designed. Because of the excellent properties of the chassis, apart from the usual filtering and level adjustment of the tweeter, linearization only has to be done in two places, one of which is even optional. With the HQ, a second-order low-pass ensures a clean drop, and an additional blocking circuit reliably levels out the baffle step. The tweeter was given a third-order high-pass filter and a voltage divider for the optimum level. The second series capacitor is unusually large at 22 µF, but still has the absolutely correct value in that it brings the edge to the woofer into the correct shape and maintains the right phase position. As an option, the linearization of the tweeter by 17 kHz can be viewed using an absorption circuit. It expresses itself at most in a minimally fresher and freer high tone. If you are looking for a monitor that is perfectly linear up to more than 30 kHz, you can safely take this measure, because it will not do any harm. The sum of the branches results in a very linear curve with an absolutely minimal increase in the treble and an acoustic separation of just under 2 kHz."
The 17KHz notch filter seems to be optional.
I'm tempted by this project myself.