Class A amplifiers - which are considered the best

The mighty Karans are most certainly up there with the very best if you're into natural sounding SS.

Goergelofif, the Krell KSA 80 and KSA 200 (and their mono counterparts the KMA 160's and KMA 400's) preceded the KSA 250 by a couple of years and were of different design generations.
http://www.krellonline.com/history.html

They still are not anywhere near pure Class A up to their rated outputs, as you think they are.
I built quite a few pure class A amps back in the 80's based around this Nelson Pass circuit
http://www.passdiy.com/pdf/classa_amp.pdf
but expaned out to 100 watts instead of 20 watts. And I can tell you for 100 watts of pure class A, the first one was bigger than your large sized coffee table, and weighed in at 120kg and still it got the heat sinks up to 60c after 3hrs and proceeded to get the dreaded class A thermal runaway, plus I had to change the power points to industrial ones because it started to melt the standard house ones.
The only way to keep it cool was to fan force x 4 the massive heat sinks, but this was too noisy. After this amp I built the same but this time I water cooled it, first one was self contained recirculated water with a quite pump heat sink jacket mini radiator and fan, it looked weird but worked a treat, wish I had photo's of it (still weighed in 60kg. After this one there were 2 more both water cooled but using the house water and draining into the garden.
This unsound is why I am confident in what I say about the pseudo pure class A amps out there, and why you should study the link to the Mark Levinson ML2 mono blocks I posted earlier and see what it takes to get 25w of A class, and they get too hot to touch

Cheers George

Georgelofi, I did see your link. I believe that at the time Mark Levinson was building those amps, he was charting fairly new territory, and those amps were overbuilt. Compare the size, weight & number of chassis, to a newer Class A amp capable of similar Class A power output:

http://www.stereophile.com/content/pass-labs-xa305-power-amplifier-specifications

I think your overall point is well taken though. Some, if not many amps that suggest pure Class A output, are questionable in that regard.. The Aragon monos, the BEL's and some of the Electrocompaniet amplifiers amongst others, do appear to be stretching credibility with their Class A claims.

Unsound: Georgelofi, I did see your link. I believe that at the time Mark Levinson was building those amps, he was charting fairly new territory, and those amps were overbuilt. Compare the size, weight & number of chassis, to a newer Class A amp capable of similar Class A power output:Unsound

This was what they needed just for 25w class A only , as they were still hot, after an hour or two idling, my friend owned a pair and I correctly biased them to spec for him, any hotter and they would have been unreliable.

Nelson Pass's favourite saying on how much class A bias heat sinks can handle, after 2hr warm up, is palms of hands on heat sinks with >35c ambient temp day and you should to handle the heat for 4 sec, this is the absolute max class A bias for those heatsinks. (brick layers need not apply for this test) soft wimpy tech hands or females only.

Cheers George