Don't know how much help this will be as I've never heard the DSD 250c or 1ET400A. I do however own a Class D Audio Mini GaN 5 (their current offering and I am using it in a secondary system at the moment but did initially use it in the main system, which is an integrated AV system, for HT duties). I now use a Purifi 1ET-7040SA stereo amplifier in the main system. It is supposed to sound substantially the same as the 1ET400A but with more power and the ability to drive low impedance loads better. Based on that and the older Class D Audio design, I'd think that the Purifi would be a nice jump in performance. Of course with anything, system syngergy matters. What is the rest of your system including electronics and speakers?