The improvement in SQ came with the new 20 amp outlet. Speaking of noise in the line, for those who grew up in the 60,s, remember you couldn’t watch TV or play records when Mom was vaccuuming? Now THAT was noise. Vaccuums motors were not double insulated then.
Your old receptacle was probably in poor shape and just replacing with a new one of similar quality would have given similar results. The "boutique" receptacles are no better than Spec-grade receptacles that cost $5 instead of $0.50 ... As for electrical noise caused by old appliances in the old days, it had nothing to do with lack of "double-insulation". Those old motors were "universal" motors, which had a fluctuating electrical arcs from the brushes to the commutator rings, and no electrical noise suppression (i.e. capacitors and ferrites) to squelch that electrical noise. Nowadays, the vacuum cleaners and other appliances run brushless motors, and always have noise suppression to prevent interference with audio gear and TVs.