Tweaks to Add A Little Treble Emphasis...

try replacing all of your speaker driver steel screws with brass screws. it will cost you about $20 and bring added clarity and transparency. this may address your needs. if not it is easily reversible and will cost you very little to experiment.
this tweak is described in detail at the Virtual Dynamics Oddiophile site. It works.

I would try some Black Diamond Racing Cones.

Is your preamp-to-amp connection balanced or unbalanced? And how long is the cable?

Assuming that your version of the Calypso is the same as or similar to the one measured by Stereophile here, the unbalanced output has an unusually high output impedance at mid and high frequencies of 2400 ohms. That will result in a slight but very conceivably significant rolloff of the upper treble, if cable capacitance per unit length is high and/or if the cable is particularly long.

If that is the case, the logical solution would be to connect via the balanced interfaces which are present on both components, since the mid and high frequency impedance of the Calypso's balanced outputs is far lower, at only 112 ohms.

Regards,
-- Al