What are the advantages to a Class A amp & what are the trade offs?

The class of the amp is only one criteria on which to make a buying decision. I have a mid 90's Krell KSA 300S that I dearly love but the Class A feature has its downsides. It weighs 185 lbs. and it runs so hot when I play it loud that I can't touch it. But it puts out 1200 watts at 2 ohms and it sounds gorgeous. It's a perfect match for my speakers (Mirage M3si) which are very low sensitivity (I routinely drive the amp into the highest bias level). This amp is a technology statement and there are very few new amps which can match it's performance, at least for under $50k. I am willing to live with the downsides of this amp because I admire the design philosophy and it sounds very good to me. It's kind of like owning a vintage Porsche that isn't very practical but it's just so fun to drive.

The Canary is a very different animal than the Krell but If the M70 flicks your Bic go for it but understand that the Class A feature is only one aspect of the amp.

@viridian4, The Krell KSA amps are true Class A. They have a feature called "Sustained Plateau Biasing." where they have a sliding bias feature that anticipates the loudest signal and then biases the amp into more Class A wattage before the signal is amplified. There are 4 bias levels and when a higher bias level is triggered it stays at that bias level as long as the signal strength justifies it. All the signal going through the unit is amplified in true Class A but if you are playing quiet music it only uses the lowest bias level. This is why the amp runs hotter when it's played loud. But it's important to note that at no time does the amp run in Class A/B.

Some amps run in pure Class a for a few watts then switch to Class B. The Krell does not do this - it only runs in Class A.

There are a couple other companies that use a similar Class A biasing feature in their modern Class A amps. They, like the Krell series, only run in Class A but don't consume their full power unless demanded by the input signal.

@viridian The Krell you refer to (KSA 250) is an older model. The KSA 300S was the next series that were designed precisely to run completely Class A unless the amp gets very hot. Here's the article that explains the difference:

https://www.stereophile.com/audaciousaudio/krell_ksa-300s_power_amplifier/index.html

BTW, with my low sensitivity speakers (Mirage M3si) I can drive the amp into the highest bias level - and it gets really, really hot. What I don't know is whether or not the top two bias lights still turn on when the thermal protection kicks in. It may be that for some of my extremely loud listening the amp goes into Class A/B. For the great majority of my listening the amp never goes past the third bias level and according to the article, and everything else I've read, the amp operates in pure Class A. I think it's worth noting that this amp weighs 185 lbs. It was obviously built to an extremely robust standard.

To clarify further, operating in Class A was the whole point of this amp's design and explains the design differences between it and the KSA 250.