First you'll want to consider if you'll ever use this upgrade on the go at any time or if it's for purely home use, and what percentage that is. If it's a low percentage for portable use, you may just want to use the CQ15 for portable use while keeping whatever upgrade you choose as home use.
Open backed headphones are just that-- open backed. They both leak a lot of sound and let a lot of sound in. However, they're usually better sounding than closed backs, because they don't have to deal with damping and resonance problems that arise with closed back headphones. Think of it as a pair of speakers playing in a square room vs playing outside-- outside will sound better and be void of any resonances the room has that would otherwise affect their sound.
It's hard to get the same type of bass extension from an open headphone than a closed headphone because low bass can't pressurize the small, sealed space between your ears and the headphone. However I recommended 3 that do a great job at reproducing good, quality and extended bass.