A21 is the originalPerhaps. But you'd need to suss it out spending some dedicated time in your listening environment with your particular set of speakers, etc. I'd guess that the S2 ("A21aMk2") would bear a closer resemblance to the A21a branch of the family compared with the SE's.
A21a is from the 90’s
A21a S2 is from the 00’s
A21a Signature is 2017 onwards
A21SE introduced around 2008
A21SE Signature 2017 onwards
Sonically, you will prefer the A21a S2 or A21a Signature with your Harbeths. Despite power discussions, your brief will work well with this combination. I can assure you that the Sugden will give you texture, midrange and clarity that the Rega simply cannot match. I’ve read what your looking for and if in doubt, try and get a demo.