Ditcin, some random thoughts. From your link its Scott I believe.
Firstly let me say the Sherbourn 7/2100a is a fine example of a terrific class AB HT amp. Its the kind of powerful 7 x 200W@8ohm monoblock amp that should form the foundation for any HT, in my opinion. I've always been a fan of monoblock designs and before the A-1400-8, it was the type of amp I would recommend to my friends who were looking for the best HT amplification. I've never owned the 2100/7 but did have a Bryston 9BSST for a while which I really enjoyed, a 5 x 140w monobloc design with gain controls to boot.
To put your question into perspective, the A-1400-8 design embraces the monobloc phylosophy and takes it a step further. If you check out the video on the amp you'll hear Tom and Ian talk about how they set out to replace a stack of 7 monos with one unit that was more dynamic, didn't clip and provided much more power than available with monos.
To get the 2100/7 to output at its max you need to plug both cables into separate outlets otherwise your limited to 1440W total or 205w max over the seven channels. Setting aside the 8th channel and dynamic handling for now, the axiom design will actually output more than the 205w max because of its ability to draw more power out of the line, supposedly up to 285w per channel all driven.
It is rare though that all channels are driven simultaneously at the same power levels and that is where the Axiom really shows its technological advantage. If needed the Axiom can direct over a kilowatt of energy into any channel. It achieves that because of how its switching circuits can route power from its 85v+ powerrails and single massive (over 1600kva !) transformer. With that much power on tap this amp will clip under only the most massive loads and demands. Despite all my attempts one afternoon with no one home, I could not get the 1400-8 to clip. One thing about monos the size of Sherbourne is that they will clip in demanding scenes or in complex musical passages.
Another main difference, aside from the Axiom being half the size and weight and running much cooler, arises from the much faster response time of the a-1400-8. AB designs take longer to ramp up and down whereby a fast switching type D amp will respond much faster to power demands. What helps make it so fast, is the way the 1400-8 has 4 x 33000 mfd caps repowering smaller 4 x 2000 mfd caps as they drain.
Slew rate can be measured several ways but the Axiom weighs in at around 8V/ms while the Sherbourne is a bit over half that rate or 4-5V/ms. I know some folks think that its inconsequential but amp designers sweat for months trying to get amps to respond as quickly as possible since it impacts dynamic presentation and how loud and sharp transients will sound.
Where I believe the Sherbourne and monos in general have an advantage is that there is zero crosstalk associated with them. Any amp with a single transformer will have some level of channel crosstalk though in the Axiom it is well in the inaudible region. Some would argue that there is more crosstalk in the DSP circuits of a processor but the less the better. The other stat I pay close attention to is signal to noise and both amps have great specs.