Can anyone share more details about FAST?
Check out this full thread at diyAudio - it covers the entire design of xrk971's original RS225-8/10F8424 FAST:
10F/8424 & RS225-8 FAST Ref Monitor - diyAudio
It's miniDSP-based with active bi-amping, as Keith mentioned. The advantage is that tweaks to the XO are SW-based, allowing for a lot of experimentation. The RS225-8 woofer is in a sealed ~24 litre enclosure. The ScanSpeak 10F8424 is in a small ~1.1L foam-core dagger TL within the main enclosure.
The ScanSpeak 10F8424 and the Tymphany TPY03W06o0111 are closely related - they were both originally designed and manufactured by Tymphany, and have similar physical dimensions. The former has a Neodymium magnet and a circular mounting-frame, while the latter has a conventional ferrite and a squared-off mounting frame. Needless to say, the ScanSpeak 10F is priced some 8x-10x more than the Tymphany, which was intended for volume consumer applications like large-screen LCD TVs and so on.
What xrk971 did, as an experiment, was to replace the ScanSpeak with the Tymphany, and tweak the XO and equalization in MiniDSP slightly to implement a 4th-order acoustic Harsch XO. He uses a TP3116 30W Class-D (with mods) to drive the FR.
The result was spectacular - pretty much dead flat from 500 Hz to 20 kHz for the full-range portion, and flat low extension down to 50 Hz and below with the RS225 woofer. The acoustic response in the highs is even better than the ScanSpeak, which rolls off around 15 KHz or so.
This experimentation works best with miniDSP, of course, but the active filters and equalization that have been employed appear to be simple enough to implement with an opamp butterworth active filter once the parameters are worked out, which is a one-time exercise. In other words, one can use the validated settings from the miniDSP 2-way crossover plug-in to derive an analog active filter design and proceed from there.
If a different woofer is chosen (say a smaller 6" or thereabouts), the same process has to be gone through again, perhaps yielding a smaller cabinet size than the 24 litres for the RS225-8. The attraction here is that a modest medium-sized sealed 2-way FAST bookshelf cabinet seems to be within reach, with a modest expenditure on an active bi-amp - both of which can be Class-D, giving further economies, but with an exceptional acoustic response from ~50 Hz to 20 kHz.