@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/