If you like the mix, just put a Utility plugin on your master chain and pull down the overall gain until you get to -6.
Also, -6 dB is mostly a relic of pre-digital (and especially pre-32 bit audio) recording and is not really all that necessary now; it's just an arbitrary value. -3 dB is fine, -6 dB is fine, -12 dB is fine... all you're gonna be doing is bringing it back up to 0 anyways, so find the level you get comfortable with. Technically, you can master with sounds hitting Unity gain (0 dB) and have it sound fine - Steve Duda has been quoted as saying his kicks will sit at 0.
That being said, it is also a good habit to try and mix quieter than it is to mix louder. Mixing is about taking away as many frequencies as possible while maintaining the integrity of every sound, and if aiming to have your mix peak at a level below 0 dB helps you to accomplish that goal, do whatever you want! If it's feeling too quiet, you can always increase the output level of your speakers if needed.
