I agree with Pene Azul
if you are gonna try though try using something like SoundForge to boost the higher frequencies
by the time you have figured how much and what corner frequency (I doubt it is a clean 6dB per octave either) you will find the signal to noise ratio has plummeted. for little time saving
If you are going to MP3 then that will be one more degradation.
If the intention is hifi it would be quicker to do real time
If it is for song learning the loss of higher frequencies is not a real issue.
what deck are you running to get higher speeds?