I use an Emotiva UMC-200 (HC preprocessor, fine for stereo too),a Crown XLS 1500 amplifier (intended for professional use) and Quad 22L loudspeakers..

The most noticeable upgrade was the automated room correction included in the preprocessor. It more or less removes the room acoustics from the equation, as well as the speakers coloration.

IMHO correct speakers, powerful amplifiers (professional audio power amplifier may be considered) and good room correction makes a great combination.