Krell FBP c versus FPB cx

I think the upgrade cost is $1K if you ae the original owner and $2K if second owner. That's what I was quoted for a monoblock upgrade. I bought three 350 CXs instead of upgrading my 250Ms. In general, most upgrades are too expensive and you are better off getting the latest version used on Audiogon.

I am mostly curious about the "C" to "CX" upgrade, financially and the perceived difference in sound. More specifically, the upgrade from the 300c to the 400cx.

There was a very similar post a couple of weeks ago, it was titled: Krell FPB300 vs 400CX
My previous post may help answer some of the questions here. Let me know if there's any trouble finding the old post and if so I can always post it here too.