I am surprised nobody has mentioned graphic equalizers. They were once shunned because they introduced both harmonic and phase distortion. It was then found that their distortion was ridiculous compared with the distortion of even the best headphones or loudspeakers, and acoustical experiments reported to have found that the human ear is deaf to phase shifts.

When listening to commercial harpsichord Audio CD's, and for any non-harpsichord CD's as well, I have found it most useful to have, between amplifier and speakers, a good 32-controls 1/3-octave stereo graphic equalizer. These marvels are now incredibly cheap (my excellent German one cost me €72). 

They can be used for recording or re-editing old recordings of course, but I use mine just by having the knobs in their optimal position for the room, which I find by using a calibrated Audio CD with pink noise ranges.
In this way the equalizer is not just compensating for unevenness in the speakers (negligible in good quality ones) but, mainly, for the very uneven reverberation of the room you are in and also for standing waves when listening from your favourite armchair. When you press the on/bypass button to compare the sound with and without the equalizer, the improvement is impressive.

Best
CDV


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