Newbie Question: VTA and Rega P25/RB600

Origin Live and Expressimo Audio both make variable VTA adjusters for the Rega arms. They are around $40. They are threaded adjusters, and not the easiest to use, but they do work. You can dial it in exactly, with a little patience and a good ear.
Generally, to adjust VTA, you start with the tonearm level while it is playing. Then, you put the arm back on the clip, and perform an adjustment, either up or down. Listen to the result, and determine if you went the right way or not. Make only small adjustments, and play around like this until you get the best sound. Different cartridges like different VTA, and you can only get it right by listening, and adjusting.

It sounds like a VTA issue to me. Lower the pivot end of the arm, slightly. If it is already as low as it can go, then cut a piece of cardboard(non-corrugated) that is about the thickness of a matchbook cover, in the shape of your platter. Put it under your mat. If this improves things, then try using two layers of cardboard. Once you figure out how high to go, then you know what thickness of the new mat will be, that you need to buy. I know cardboard may not be the ideal material to do this with, but we're not looking for a permanent solution with this, just trying to determine how much spacing you need for your VTA adjustment. You can then get the right thickness mat, that will be made of the right material. If you would rather use another material than cardboard, that's fine, just so long as you get the spacing result that you want.

The other option would be to buy pre-made spacers to put between your cartridge and the headshell. This is alot more difficult, but will achieve the same end.

Also, the 10x4 is designed to work into your MM input on your phono section. It is possible that the loading on the MC section may be set at something other than 47k, which is what the 10x4 is designed to work into.

Oh well! And not one of us even suspected that. Glad you got it working.