Usually how it works with SR is that the sound signature is pretty consistent but you get “more” of each sound characteristic as you move up the line. Each time they introduce a Galileo version of a new line, a year or two later the lower priced SX version delivers near the equivalent sound quality. Kinda like Sony TVs where they introduce a new TOTL XBR model and the next year the cheaper Bravia version has most of the prior year’s XBR technology and performance for a much lower price.
Specifically regarding the Powercell products, pay attention to the model of SR power cord that is paired with it. Opting for the Galileo SX power cord upgrade with the lesser Powercell will get you very close (or equal?) to the upper model when equipped with the standard Atmosphere power cord. Of course, the top model with the better power cord is supreme.
I usually buy two years behind the latest model on their Powercells and always opt for the better power cord, both purchased used. For example, I now have the Powercell 12 UEF SE with Galileo SX power cord upgrade and may move up to the Powercell SX at some point soon. This method keeps me moving up perpetually at a much better price.