In the article, Morgan Jones lists a characteristic of SET amps as "...Non-zero output resistance – typically one third to one half load resistance." Parabee would be very close to the one third point. It's mostly the 300B's plate resistance, with the transformer's winding resistances added, all normalized as appropriate by the turns ratio.
In the diyaudio discussion (I only glanced at it) someone calls attention to the similarities with the S.E.Xy Speaker design on the Bottlehead pages:
http://www.bottlehead.com/loosep/S.E.Xy%20speakers.html The main difference is the S.E.Xy used the FE166 which has a lower Qt, leading to a smaller box than the Arpeggio which uses the FE167. The FE166 is a bit more efficient. Both drivers appear to use the same cone and voice coil, and would thus be expected to have the same high frequency performance. The main difference is the magnet structure.
It was our experience that the 6.5" Fostex drivers needed the bit of treble boost that is provided by the rising impedance - a high frequency Zobel as described in the article would actually reduce the treble. In fact, we used a modified BSC to add some more treble boost. Of course your mileage and taste may vary - trust your ears when deciding what you like!
