Of course you can add a tube device, like the MF X-10D mentioned by a previous poster. You can also add some type of tone control. However, looking at the art posters you have in your audio room, it makes me wonder if the problem isn't the media you choose to play.
Lots of rock music originally intended for vinyl sounds horrible on CD. Do you always feel the need for tone controls in your system, or is it just when playing older rock CDs? Do you ever play anything where you don't hear the need for added musicality?
Just curious as I have lots of older CDs (and some newer one, like U2) that are nearly unlistenable on a high resolution system. They do, however, sound just fine played on my car cd player!
If the issue is making bad recording more listenable, without affecting your good sounding CDs, maybe you could add a musical/colored DAC between the CD player and pre-amp. You could have the analog output of your CD player and the analog output from the DAC connected to different inputs on your pre-amp. You could then "engage it" as needed by simply selecting the appropriate input on your pre-amp.
Enjoy,
TIC
