Normally, an Audio Technica AT7V and a Rega RB300 tonearm are not a very happy sonic match. It's not really bad, but the music seems to be lacking the proper foundation, and on the slightly hissy recordings the sibilance is highlighted. The stereo image is flat and seems to "cling" to the speakers. And the magic just isn't there. AT7V doesn't sound materially better than cheaper AT cartridges in the stock RB300.
With the AT633 spacer, the differences are immediately audible. The improvement in bass is expected and very welcome. You can put the numbers into our compliance calculator
and see that adding mass to the Rega RB300/AT7V combination brings the acceleration line into the green zone (add 1.75g in the "headshell" field. RB300's effective mass is 11.5g, AT7V weighs 6.5g, my best guess at its real world compliance is 12-15).
Sibilance is not just diminished, it's completely removed. With the spacer in place, one can finally appreciate just now neutral
the AT7V is. The imaging is also a lot better, and some depth can be felt. Unfortunately, the tendency to cluster the images close to the speakers remain.
That 850 Hz peak? I didn't hear it. RB300 has some coloration in the mids, and I think it masked whatever changes this peak might have brought.
To sum it up, very impressive improvement for something so simple. An effective cure for RB300's main aliment. And the spacer should be equally effective with any headshell that can benefit from more rigidity.