33 1/2 compared to 45 rpm ????

Osgorth has answered this well from my experience. The 45 rpm disc, all else being equal, moves a fairly significant step closer to what I would expect from the sound of the master tape: higher resolution, better image focus, improved clarity on the inner grooves. I hear this over and over again on the LPs I've compared where I have both a 33 rpm and a 45 rpm of the same material from the same mastering engineer.

For a quick discussion of why, read the comments from mastering engineer Kevin Gray at Music Matters.
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