I had a nice conversation with the echo rep when he was in our local shop a few months ago. One of the things I asked him about is why Echo doesn't publish there HP ratings and he told me they dont test HP because there is to many "variables" when testing and its just a number.
I also asked him about the 590 advertising it makes 4hp and he said that spec was to compare the timberwolf to the Stihl and Husky farm and ranch saws.
I haven't ran a 590 or 600 yet but judging by the build and the reviews i would take that over ANY Stihl or Husky farm saw.
I would be curious to know how different the cylinders are between the 2 because my dealer has them marked down to $399 with no sales tax.