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

Your Sudgen is a respectable amp, and have worked on some Altec related custom designs with a few friends, they can be transparent with great crossover parts inside, once upgraded.  While I hate to open this can of worms, what sort of source and cabling are you running for ICs and speaker wire?  

Are you already running silver plated interconnect or speaker cabling,  sometimes a simple move from OFC to OCC crystal copper cabling can add transparency.  OCC can be really open and smooth too.  Maybe some other areas to check too while considering replacing your amp, fwiw.  

@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/

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.

Extra long speaker cables and extra long interconnects with high capacitance can be impactful. Particularly as noted earlier when gear/voltages are not designed to be used with long runs.   

A simple test is to pull all the gear super close together in a room, even with short low grade cables, make everything super short (for a test).  Listen and see if you can hear any difference.      

@alaindexe
next step, the cross overs...

FYI, for some added inspiration - peeking over on the DIYaudio site, members were commenting about before and after results from upgrading and refreshing the old Altec 604/620 crossovers with GPA crossovers and replacement parts. Quote, "it added a sense of spaciousness and clarity to the speakers", unquote.

@zm
Which FirstWatt model are you comparing to?

Each of them have a different circuit topology and output transistor types, SE, PP, different impedances, and wattage power output. Interested if your AA Pucciini sounds more like a SIT-3, F6/F7, or J2, etc. Also dependent on what preamp you are using too. Thx.

alaindexe’s Altec 620A speakers are close to 100db sensitivity @1w, 4-feet away, right?. Can operate well with two (2) really good watts. Low watt delicacy of targeted 20-30w musical amplifiers like Firstwatt, Sugden, Pucini sound would be the ticket for these speakers.  Quality watts needed more than quantity watts in this case.  

@ebm
Audia Flight Stremento #4 mk 2 sounds like the best class A plus its very musical with plenty of power.

Good grief, is that 28 Mosfets (14 per side) in that Stremento #4? 250/500 watts? https://www.hifinews.com/images/719audia.ins.jpg

@alaindexe
Any thought on the Audio Sector products (gain clone) ?

I’ve never built one and read up on some stuff Darko was doing years back. Reviewed the DIY LM kits, not much help to ya. Maybe check the DIYAudio site, folks there can possibly help a bit more on that one. Good luck.