Airwindows has released SlewOnly and SubsOnly, two effect plugins that let you hear only the highs, respectively the subsonics, and adjust your mix accordingly. They can also be used as special effects.
Mix monitoring tools for VST and Audio Unit, Mac and PC. Please remember, these like Guitar Conditioner are control-less plugins so you don’t operate them by opening their windows: they’re either on or off, which probably is shown in your DAW mixer window already.
SlewOnly gives you what it says on the tin: only the difference between samples, expressed as a sample. This produces a super-bright sound with zero latency and absolutely no pre-ring or post-ring. You can do two poles of this filtering: just add another SlewOnly. But, as is, this will give you an incredibly clear and transparent window on the ultra-highs, so you can dial in subtleties in the treble. Beware: don’t switch right back to the normal sound or it’ll sound incredibly dull. Rest your ears for a moment before resuming work.
SubsOnly does the same thing for subsonic bass. It sounds a little like a house party from an adjacent house. You can tweak sub-bass elements and really hear how they’re balancing against each other, and if you’re packing too much into the subs you’ll hear that as well. Get things thumping properly through SubsOnly and it should translate well to all manner of bassbins and subwoofers.