I had the A21 for 3 years, and currently own the Adcom GFA 565SE, 555MS, and had the 555SE. The Adcoms are all current production models.
I didn't have the Parasound at the same time as the Adcoms, but sold it because I grew dissatisfied with the way the top end is lightly rolled off. Unfortunately I never tried different cables then because I believed the "cables are snake oil" nonsense. With my speakers and pre, I also couldn't get plentiful bass, so that was strike three. I think the gain structure had something to do with that.
I believe the A21+ would be fine, any amp just needs good synergy with the rest of the system. The gain pots, signal sensing turn on and good looks don't come cheap. But the Adcoms are a lot of fun at half the price. Made in Taiwan as mentioned above, same as Parasound.
The 585SE is the only one that has the 12 volt trigger if thats a priority.