I listened to the Paua many times mounted on an SME 20 TT with both the Soundsmith phono and Linn Uphorik - both solid state, but have the ability to load up to 1000ohms.
As I stated in my earlier posts it requires loading between 500-1000 ohms.
The Tavish has Jensen transformers for the MC gain which I suspect limits your loading to a max of 470ohms, and I believe the source of your problem is the use of SUT's ( in the Tavish ).
I think you will find out if you try the PS Audio or Soundsmith phono or a suitable phono that provides loading options 500-1000 ohms withut the use of SUT's, you will be better off.
The other issue that may be causing brightness, might be VTA. Since the Rega tonearm has to be shimmed to get the VTA correct - has this been attended to ? If you can measure the height of the Paua and compare it to the height of a Rega cartridge ( from top of cartridge to stylus ) this will give you an indicator of whether you need to adjust VTA.