I have compared the buckeye amp thoroughly, including the REW graphs, and decided to give my A21 away to save space and before I get too old and weak to move that thing around.
View attachment 72448
Those 60 A current specs are silly because they never tell you how they measured it, and it might have been for some microseconds, really meaningless. All it says is "60 amperes peak per channel"
Harman Kardon and NAD are probably among the worse in perpetuating this so called high current capability myths, for example, Harmon's ex flagship AV receiver, yes, just an AVR that is rated 110 WPC into 8 ohms, has the high current spec 75 amps, taken right from the owner's manual:
High Instantaneous Current
Capability (HCC) +75amps
Does that mean such an AVR more current capable than the much bigger and heavier 2 channel Parasound, of course not, by the same token, you can based on the A21's spec and think it is more capable than the buckeye NC502MP amp that simply says:
Hiccup both channels after
limiting 40ms
not all that informative and such specs are not really relevant in real world applications where 500 W into 2 ohms means 15.8 A, or 22.4 A peak, that's well below the current limit set point of the buckeyeamp NC502MP.
The Halo A21 is only rated 400 W into 4 ohms, so current at rated output is only 10 A, peak will be 14.2 A.
The NC502MP is 2 ohms capable, whereas the Halo A21 is only rated for 4 ohms, though it can of course deal with impedance dips down to 2 ohms, just not rated for 2 ohms nominal loads.
Anyway, if you are interested, you can browse through the thread linked below, a poll was included:
Class D power amplifiers | Audioholics Home Theater Forums
The buckeyeamp is budget friendly, if you don't mind spending more, take a look of the Apollon amps:
Apollon Audio - Hypex NCore, Purifi Based Class-D Amplifiers
Any of those Hypex or Purifi based class D amps will have much lower distortions and noise than the Parasound A21 and your current Krell amp for sure.
The A21+ has a little more rated output but for low impedance loads especially with large phase angles, the Hypex or Purifi based class D amps are more suitable.
The STR preamp seems like a good match with those amps too.