Hi,
Recently I posted an item where I had a significant win in overcoming the Muddy Mix... (HERE)
But I've found an additional approach which is another step forward in achieving a clear sounding mix when reviewed on multiple speaker systems.
 
I guess the pro's already know about this, but this place is a forum mainly for the home studio muso always eager to find improvements to technique, and this is what I've found.
 
Being a composer and singer of primarily Ballads with acoustic guitar as the main instrument, and also having a voice in the lower registers, I found that some vocal notes would sound odd when the same note was playing on the guitar.
The use of the High Pass filter has been great, and I normally slide the cutoff frequency upwards until there's a nice balance on my 3 speaker systems as mentioned earlier.
However, with some songs, this clash of lower register vocal notes is sometimes tricky. I have been sliding the cutoff frequency upwards to reduce this clash of vocal & guitar notes. But then I found I was losing bass tone from the vocal by having to push the cutoff frequency to far up.
So I then bought it down to a point where you might say it's too boomy and should have more bass reduction, but I incorporated a notch on the Boomy frequency by the use of a Dynamic EQ plugin. (Tokyo Dawn's NOVA EQ).
 
 So, I have successfully reduced the clash of vocal/guitar, and preserved some nice bass components to the vocal by pulling back the cutoff frequency of the HPF, and adding a dynamic notch at the appropriate frequency.
 
Cheers,
Adrian.
 
PS: NOVA Dynamic EQ is awesome and FREE!... (and a new version was released mid November 2016)
 

Sometimes one bass note can be louder than the rest and it is worth setting up a very accurate notch at just one frequency.
 
For a whole mix though there is often build up around 300 Hz. So its nice to sometimes insert a dip around 300Hz. About a Q of 2 or so and 3 to 4 dB maybe of attenuation around there. This will often clean up an entire mix super quick.
 
Mixbus has one of its tone controls centered right in that area on its master buss. Just one little tweak anticlockwise seems to change a whole mix for the better.

It's probably worth trying a low shelf as well in situations where high passing is too extreme. Sometimes you just need to attenuate the lower frequencies, not roll them off.

I find making EQ holes for each "Instrument" (Counting vocals as instrument).  Works well.
Graham Cochrane gives a nice demo here:
http://therecordingrevolution.com/mixing-with-eq-carving-eq-holes/
 
In fact the entire series is good.  So is the web site!

sharke
It's probably worth trying a low shelf as well in situations where high passing is too extreme. Sometimes you just need to attenuate the lower frequencies, not roll them off.

I was thinking the same thing.   acoustic guitars sound kinda wierd when recorded with too much boomy bottom end in them anyway.   From across a room (or table for that matter) the low end naturally dissipates and you hear more mids and highs normally.  When we close mic it too close to the sound hole  that needs to be attenuated to sound right.