On the amp side the replies above are right — modern AVRs in this class sound the same, and “bad transistors” isn’t really how it works; what differs is features and room correction, so Dirac on the MRX740 is a genuine practical upgrade. The bigger question is the sub comparison: the MK V12+ is a different output class from the NS-SW200, mainly in extension and distortion below 30Hz at high levels. If you listen loud, that’s where you’d actually hear it; if you don’t, the Yamaha with careful placement and calibration is fine. I’d put the money into the sub and room treatment before worrying about the AVR itself.