WIth the lack of tone controls on most newer gear, a simple rotary knob adjustment is nearly past history. You can insert a Behringer DEQ 2496 in the digital pathway. It has a parametric EQ with various slopes that may tame it for you. Used strickly in the digital domain, it's nearly dead silent.
Another idea is assuring your speakers are slightly off-axis from your main listening location. Gain can be very sensitive to producing harshness, too. Be sure your gain never "clips" - that will add much more graininess to your highs, plus, possibly damage your speaker drivers.
Some systems are more digital friendly, too - such as tubes. Certain types of speaker drivers are easier on the ears in high frequencies and upper midranges.
Screen your disk purchases carefully, too. Many are recorded with the gain set way too high. Carefully select the best recordings. Optical glass cables like Van Den Hul are a bit smoother in my system compared to digital RCA or AES/EBU connections, too. Some have had luck smoothing highs by changing to silver speaker cables or adding a short length to their current cables.
Cables can contribute, too. Check that your using the proper analog and/or and digital cables in the recommended 75 ohm and 110 ohm versions as needed. Some non-audiophile connector treatments can turn into insulators and really cause havock on highs, too. Cleaning connectors with 90% alcohol and a super-thin, careful application of DeoxIt Gold has worked well for me.
