I own the Herron M-150s. I have never heard the Spectron Musician III MK2 or Classe CA-400, but, like you, I am intrigued by the Spectrons.
Here's another thought for handling your power hungry woofers: bi-amp the Herrons, if your speakers have the necessary binding posts. A reviewer for one of the audio ezines went this route and told me that another reviewer he knows of did the same thing. It would be cheaper than the fully upgraded Spectrons, because used M-150s can be had for $2,000 - $3,000 a pair. I would have done this myself, except my speakers have an almost unworkable set of binding posts that are a PITA for just one pair of speaker cables.
The Herrons have been known for a lack of lower end umph from the beginning. I remember reading suggestions to supplement Herron amplified systems with a subwoofer.