I can't comment on the DartZeel as I've only heard them at shows. I can, however, comment at length about the Ayre MXR - KXR combination, as I've had the monoblocks for almost 2 years, and the new preamp for the past several months.
The monoblocks are, IMHO, very deserving of the accolades they have garnered. They truly combine the best performance features available from world-class tube and solid state amplifiers. I am aware of quite a few industry stalwarts who have made the MXRs their reference amplifiers (several of whom had been die-hard tube loyalists). An important benefit of the MXRs vis-a-vis the DartZeel amp, is the fact that being monoblocks, they can be located immediately adjacent to their respective speakers, to keep to an absolute minimum the speaker cable length (which has a very significant positive impact, all other factors kept constant).
While the MXR was a breakthrough product for Ayre, the KXR takes it to another league. I really believe the KXR is the most significant product Charlie Hansen & Co. have yet designed - a defining product. On the one hand, it is the most expensive audio product I've ever bought, on the other hand, it very well may represent the highest value per dollar spent. To put it another way, these components will certainly be my reference preamp and amp for the next decade (or two).
Effortless and musical are the best way to describe them. The dynamic range available regardless of volume is a true revelation. While I believe there is a synergy between them, I don't believe they need to be used together to show what each can do -- any system could benefit from either component separately, or both together.
Feel free to e-mail me if you would like to know more. - Pete -