The Hafler amps, with their MOSFET outputs, tended to be bass-shy audiby and measurably, and did not have real good (high) damping factors. That made them a good match with the tube gear I had at the time, but less than ideal for driving dynamic loads with lots of charge kickback. The servo circuit took care of most of that in my design (a simple LDI feedback using the second voice coil, nothing fancy -- this was during my college days). There were some other minor issues, including a little hysteresis in the electrolytic input coupling cap (solved by using a better cap) and minor wiring issues (tweaked the gauge and dressage).
An interesting aside is that a friend had the same amp in his system (not bridged) and we heavily modified it with new caps, bus bars for power supply wiring, etc. All our worked helped a
little bit... The thing that made BY FAR the largest difference (audible and measured) was to build a bank of power supply caps in an external box wired to the amp through very heavy cables. It turned the relatively anemic bass into a powerhouse. Bear in mind at the time the other amps we were comparing including a Phase 700 (mine, a love/hate affair) and various ML, Krell, and Threshold SS amps. We did not achieve the same level of performance, but awfully durn close, and for a fraction the price.
Back on-topic: The miniDSP has mixed reviews on AVS but a couple of other sites (which I did not bookmark; need to not stay up so late on Saturday nights staring at this little bugger!) were quite favorable as far as SQ goes (bit of a surprise, but I'll take it!) One of the drawbacks seems to be the low'ish output level, 0.9 Vrms for the basic version. The miniDSP forum has comments from the designer(s) saying it is due to the USB 5 V supply limit; reasonable, but I do not see why they don't provide a higher-output 9 - 12 V version -- it can't be that hard! I need about 1.2 V to drive my power amps to FS so would get the balanced version, with its strange connectors, or step up to a bigger unit (more costly, and not available in a box so more work). Annoying... I am contemplating getting one and customizing the output opamps' gains to provide a bit more output, understanding I will use a 12 V wall-wart to power it. Being able to tweak and play is just sooo enticing...
The Marchand units seem to be almost universally praised, but $300 for a few coils in a box is irksome. Worth it, given the price of everything else, but makes me think I should just build my own. In the spare time I do not have... I would also prefer a standard Butterworth for this application since the AVR sill compensate the phase match and I would like faster roll-off around the crossover than a L-R design. Plus the sub has a phase control knob as well, so in this case I don;t really need L-R for the HPF to the Maggies. OTOH, L-R is perfect for bi-amping the Maggies later, assuming I decide to buy another Marchand (or whatever) for that.
Hmmm.. Wonder if my wife would allow me to pick up another XPA-2 during their winter sale so I could bi-amp my mains? I don't really need it, but it would sound cool, and there're all those clothes she buys and doesn't use. I could point that out, but hospital bills would really crimp my audio budget...