Doug, thank you kindly. My comments are as follows:
First, as I know Doug realizes there are two different test standards against which rated cartridge outputs may be specified. If I recall correctly that can produce a disparity of +/- 3 db between ratings that are provided by different manufacturers. 3 db corresponds to a voltage ratio of 0.707, so it is possible that the output of the Rega cartridge, nominally rated at 7 mv, is no different than the output of some other manufacturers' 5 mv cartridges.
Second, early on in this thread I had asked "is the harshness and thinness particularly or mainly evident on high volume peaks?" My understanding is that the answer was "no." Therefore I agree with Onhwy61 that the problem is unlikely to be related to phono stage overload.
I also inferred from the answer that the combination of cartridge output and system gain and sensitivity structure is not such that the volume control has to be operated excessively close to the bottom of its range. "Excessively close" meaning that volume adjustments might become overly coarse, and/or that the two channels might tend to get out of balance.
Assuming that my interpretations have been correct, I don't think that the relatively high output of the cartridge is an issue at this point, although if you were to change to a different cartridge it would be preferable to select one having a significantly lower rated output. Which shouldn't be hard to do, as I am not aware of any other cartridges rated as high as 7 mv.
If you'd like further commentary on the overall gain structure of your system, let us know what amplifier and speakers you are using, and what your listening distance is.
Best regards,
-- Al