Would the 300W CJ ART 300 Monoblocks be too much for my Avalon Ascendants?

Omg no. Sound like a great choice.

The power recommendations are… well, nearly meaningless. I have run amps of greater than recommended for decades. Either choice would be good. But in this case moving from Sonic Frontiers to CJ is a big move in sound quality, so great choice.

Two amps that I recently purchased are stellar.  The PSAudio BHK 300 hybrid monoblcks and the Atma-Sphere 100 wpc class d monoblocks.  Just in case u are open to suggestions....

I tried over 15 amps on my aerial acoustics 20t, ranging from berning, to levinson,pass labs, clases, to ayon, to luxman and many others. And then  finally the conrad johnson art 300s. The different amps all excelled in one area or another, but usually had some noteable shortcoming. When i found the art300s, they finally gave me EVERYThing. Soundstage, detail, smoothness, and bass control that many other even solid state heavy hitters could not match. I don't think about amps anymore. 

I tried over 15 amps on my aerial acoustics 20t, ranging from berning, to levinson,pass labs, clases, to ayon, to luxman and many others. And then  finally the conrad johnson art 300s. The different amps all excelled in one area or another, but usually had some noteable shortcoming. When i found the art300s, they finally gave me EVERYThing. Soundstage, detail, smoothness, and bass control that many other even solid state heavy hitters could not match. I don't think about amps anymore. 

Should be a nice pairing.

The  CJ  ART 300 will be a very musical and rich pairing with the Avalons - my kind of sound - and a large improvement from the SF.  I  was an SF fan once having a Processor 3 and Iris transport but with amplification the CJ will be better.  Until my current system, I ran amps that exceeded speaker power specifications with no issues.