I lived with the 450 MCX for several years. When I exchnaged it for the Evo 600 (now the Evo 600E) I did not notice a significant difference (I didn;t A-B them at home). Midrange,bass treble all sound great with both amps. Both have excellent 3 dimensional sound stage, instrument seperation (air). My dealer felt the treble was a bit more prestine with the Evo amp, but I am not sure that I agree. The sound is too close to compare without direct A-B. My A-B comparison at the store was a Krell Evo 400 stereo amp A-B'd to a Krell 750MCX stereo amp. The sounded very similar. i guess it is impressive that the smaller evo 400 could match the 750MCX. T really had to strain to detect a difference. I would not trade in the MCX for the evo amps unless you get a great deal. Both are superb. The thing that made a huge difference in audio performance in my hifi was changing my Krell KCT pre-amp and Lexicon MC12B processor for the Krell Evo 707 processor. This made an obvious enormous improvement in all aspects of audio both for music (2 channel stereo using the pre-amp bi-pass in the Evo 707) and for home theater. I noticed it immediately, no AB comparison was necessary. I have been involvrd in this hobby for over 30 years. Upgrading to the Krell Evo 707 is by far the single greatest upgrade I have ever done. The 707 uses a pre-amp section very similar to the preamp used in their top of the line Evolution 2 (retails for $55,000)in all channels. The sound is absolutely stunning. The Evo 707 is amazing, it completely blows away the Krell KCT and the Lexicon MC12B.