Sunfire 300 vs Signature 600

Is the difference only that the 600 has more power? Or is it sonically superior in other respects as well?
yeas...both have the bob carver character.
Let me re-phrase that: Aside from the difference in power, is the Signature 600 a better amp than the 300 in any other respects?
only if you need the extra power. otherwise no

Well, to answer my own question: Having recently purchased two used Signature 600s online for cheap, and replaced the 300s in my system (one amp per channel with Marigo Ultra 5 power cords), I can say the 600s are a significant step up from the 300s.

Imaging is more clear and three-dimensional. Less distortion. Even greater detail. Tighter and truer response from bass to treble. Simply phenomenal performance.

It took a long time for a friend of mine to convince me to try the Sunfires as an alternative to the Atma-Sphere MA-1s I had in my system since 1991. (SS gear? Not for me, thanks!!!) I'm glad I finally did.

Setting aside all the "non-musical" advantages of the Sunfires (runs cool, virtually maintenance-free, no warm-up time since they can be left on all the time, the price for one used Sunfire is the about the same as the cost of matched tubes for the Atma-Spheres for one year's worth of listening, no fiddling with bias and dc offset settings, etc), the sonic performance is superior by a wide margin.

Now, I'm sure the MA-1s have been improved since the early days, but, thanks to a marketplace that generally views the Sunfires with disdain, I for one would not be willing to spend the 4-5x needed to see how they compare today.