The PM220-8 has comparable efficiency and has considerably lower Mms and Fs than the 420.
If you are crossing over at that point then the minor 2nd harmonic peak around 1.5 kHz for the PM220-8 won't really be a factor. (..and non-linear distortion is a fair bit better with the PM220-8.)
Driver linearity with DSP won't be a factor either.
Stored energy decay could be as far as clarity is concerned, BUT a CSD plot can be read somewhat differently. A lot of what counts as "clarity" is just how fast a driver can drop just -6db after its initial pulse. Lower Mms drivers tend to do this, while at the same time (negatively) reacting more to the resonances from the surround resulting in longer decay below this spl.. This is something you have to listen to in a variety of cabinets (with regard to loading and air-losses to the driver's stroke in particular) to see what you prefer. I usually prefer the lower Mms driver in a larger vented chamber (with the vent freq. well below the high-pass crossover point and NOT designing for freq. extension), and NO stuffing in the enclosure. This sort of design tends to produce better clarity and low-level detail.
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