I'm trying to work out the correct mixing technique for fitting sounds that have broad frequency ranges best in a mix. Sounds such as pads, played as chords, can cover a broad part of the spectrum without any noticeable peaks, and an instrument like a piano, when playing a range of notes and chords gives peaks and troughs right across the spectrum as the song progresses. I've learnt a lot about carving space for sounds in a mix, and its relatively easy for example bass sounds which cover a small frequency range, but not so easy for the examples I gave above.

Any suggestions welcome.

I think of pads as grout. They fill gaps. But as with grout, you first slather it on and then remove the excess.

Usually, that starts with the low end, applying a HPF somewhere between 200 and 400Hz. Then I move to the upper mids (1-4KHz) where the clarity of lead voices lives. I put in a large boost (some equalizers offer a solo-band mode which works great for this) and sweep it up and down until I find the band where the pad clashes the worst with the lead vocal or instrument, and dip it there.

Even more effective is to simply not use pads everywhere. Holes are just as important as notes so you don't want to fill them all in. A pad might only be needed in verses and not the choruses, or the other way around. Or you may just automate the pad's level so that it backs out in those areas where things are getting too busy and thick.

Another powerful technique is automating EQ on pads. Instead of putting a static HPF on, automate it so that the pad's low end comes up in those thin spots where you need some extra grout.

Try to play your piano at different octave.

If your pad chords are at 440 Hz, you can play the piano at 880 Hz.