Vandersteens method of having a capacitor in line to roll off the low frequencies was also done very early in the Dahlquist DLP1 crossover.
In it you could change capacitors/resistors to suit your impedance and roll off frequency. Not sure if anyone else implemented a similar approach.
If you are at all handy with a soldering iron you could pop the top on your amp and install a pair of caps inline on the RCA + inputs. This would roll off your amp at a chosen frequency. Say 80Hz or so. Then your powered subs fed from your pre can go down from there using their built in crossover. The crossover calculations are available online to determine the size of caps and (resistors) needed.
Vandersteen rolls off the entire frequency band, (via MIT caps in the pair I had) sends it to the subs and then has a boost circuit in the subs power amps to regain what was taken away, while the main speakers are then passively being fed (with roll off intact) thru the subs connections.
Works fine, just a different way.
Nowadays digital processing seems to be the preffered method via devices such as MiniDSP.
