The use of the EQ’s in a DJ set remains a subject of much debate.  Some DJs are do fine without even touching the rotaries, and other DJs constantly tweak them as if they were a volume control that needs constant fiddling.  Hip-hop DJs will rarely use the EQs in a song, but they have no problem taking out the bass line of one track while slowing injecting the bass from the incoming track – this is a fairly standard practice.  Some DJs use their EQs to create faux effects, like boosting the mids and killing the highs to create their own sort of mid-pass filter. What I have discovered is that if a song is EQ’d correctly when it was produced, there is little need to tweak anything at all.  Neither bass line will overpower the next if this was done.  Mastering involves taking a look at each instrument and figuring out where it needs help, since the stock instruments sound rather lifeless and flat without the right types of effects.  Compression is commonly used to add punch to the attack of a note and can even be used to brighten the sound of a digitally created synth or keyboard.  http://www.attackmagazine.com/technique/walkthroughs/how-to-eq-mix/