AlumoAudio, adammonroe, what monitors do you use?
Hey Permian, I'm using an oldish pair of Alesis Monitor Ones. Perfectly happy with the sound, so never got round to replacing them!
Out of curiosity, why do you use speakers instead of headphones? I have a pair of sennheiser HD280 Pros (The voice actors on family guy use these) and I don't think you can replace the feel of your head vibrating from heavy bass.
I actually use a pair of AKG K271 Studio Headphones too whilst recording, arranging and proofing, but mastering should always be performed and committed through monitors.
Not to go off-topic here, but a mix may sound great, powerful and bassy through headphones, but as soon as it's played back through speakers, can sound awful, muddy and full of artefacts that even the best headphones can't reproduce. It's a simple issue of soundwaves pushing the air from the source to the listeners ear.
If you can make a mix sound great through your monitors, chances are it will sound acceptable elsewhere too. It's also a good idea to reference your mix on a variety of outputs.