Bigkidz's description of the differences is very good, though it was the early, now discontinued PASS amps that were hailed as sounding musical in a tube-ish manner - and that was a long time ago and now these amps are considered to sound somewhat veiled (though based on a kickin' design, both electrically and visually, there is no doubt still a future for these classics).
However, when picking a new amp,there are a few caveats:
In a well-balanced system the EDGE will win hands down, everytime, including in the areas of naturalness and note decay mentioned above. They have all the advantages of solidstate but do not sound harsh or... solid-state-ish.
But, for example, if your system is sounding a little muffled, then the PASS X-series will help compensate, and help balance out the sound of the system as a whole. I would suggest this is a house of cards and leads to some amount of fustration over time as the weak components can make any added superior components sound 'bad'.
Another perspective is that, if the some other part of your system needs serious upgrading, then, for example, a $4K preamp + $6K amp may very likely improve your system beyond that contributed soley by a $10K amp.
EDGE does make less expensive amps that sound very much like their big brothers. If you care about the amp not sounding ... one might say rude and obnoxious and overly agressive... while at the same time being transparent and not artificially sweetened or muffled - and you want to use solidstate, then in our experience you have to look at the EDGE, HALCRO & HOVLAND (for certain easy-to-drive speakers), LAMM & TENOR (solidstate hybrids), and from what we hear from our overseas friends, perhaps the DARTZEEL and FM ACOUSTICS amps.
-Mike (Dealer for EDGE and LAMM who had a very fustrating battle with a house-of-cards system in his youth for waaaay too long)