suggestion for a SS class AB amp that sounds like a class A ... if such a thing exists

You can get a used Unison Unico Due Integrated for under 2K.

https://powermodules.com/aria-integrated

@alaindexe
@decooney : it seems that you were right... (read above)... thx...

alaindexe, 
Thx, we will see. Said the old way, "the proof of the pudding is in the eating".

Disclaimer:  Assuming your current amp is factory original Sudgen and not a copy clone, some have copied that circuit in the A25B, read on if interested.  

"Transparency" and "details" is the Goal, correct?
Was thinking before you move past the cool Sudden A25B amp, are your Altec 620As original, 43 years old? Old dried up caps in the crossovers? Time for updated crossovers, maybe? Speaker surround stiff, maybe needing a refresh or proper update. As many like to say, start with the speakers. It applies here too, worth re-checking.

Contact these guys, maybe send them your crossovers inside your speakers or send your crossovers and the speaker drivers, both. If you are not sure, have a local shop pull it all for ya, box it up, ship it out. Hey, gotta tell ya...You might love that "musical" Sudgen amp all over again with some other upgrades first starting with your speakers, then some good speaker cables and really good IC interconnect cables after, wallah! A modern speaker crossover update, with really good parts can be transforming for a 40+ year old speaker. Followed by great cables after.

If this does not interest you, and you "MUST" jump right now and go buy another amp, send me a message, maybe I’ll take that Sudgen amp off your hands so you can go buy another amp - which might NOT help gain you more transparency. Maybe start here as option-B, just a thought. Root cause analysis can truly pay off some times, start here just for grins :) https://greatplainsaudio.com/jeff-markwarts-corner/

@decooney : yes it’s an original A25B... and I just change the Alp potentiometer (with an original part - no worry)... I will eventually have to change the caps on this one...

The Altec 620A is the enclosure model... speakers are 604 8G (Alnico magnet) ... never heard of surrounding becoming stiff with that kind of speakers... but the cross over needs work, that’s for sure... I’ve wanted to upgrade it for quite some time but it’s quite an investment.... I know of Great Plain Audio, thx...
The Sugden is a great little amp... As I mentionned before, it does play quite well with my vinyl set-up... but it still isn’t as transparent as my previous amp, a CM lab 911 that I got rid of because it required too much maintenance (I had it entirely rebuilt twice). I bought the Sugden because I need an amp until I got something to replace the CM lab but end up keeping it... Even if I were to get something else, I’d probably keep it as a back up amp...

thx for taking the time to kick my a--... and sorry for not selling the Sugden : )

No worries, @alaindexe , all in good collaboration - you have a nice amp and speakers worth investing in. Fwiw, a few years back, helped a few locals to spec/design/build some very large Altec (Onken, custom) fridge size speakers (3 pairs). It was another new learning experience, before & after results with updated crossovers and GPA updated drivers, along with some other JBL drivers in another project. Listened to two pairs driven by 2-watt to 8-watt true class A amps. The results were beyond expectations. Much of the credit went back to the upgraded speaker components.

The Benchmark will be by far and away the least class A-sounding, so delete immediately from your list.

That's an interesting comment, since Class-A "sound" is far more influenced by design goals. A low feedback Class-A will have much different distortion products versus Class-A with more feedback, and that is not even taking into account voicing.

If someone makes a transparent sounding Class-A, i.e. neutral, not voiced, and no intentional distortion, then it would be closer to the Benchmark who has very low distortion at low power, somewhat negating any advantage of say Firstwatt and the justification for most ClassA, i.e. removing zero crossing distortion.