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.