I've been joking about this one on another forum for a bit. Cotton swabs and cleaning your ears is a great system tweaks. The rumour is that manufactures are lining up to market high end cotton swabs and the 'C' rating standard for cotton purity is being established.

But seriously, your ears are part of your system and back when I took recording music a little too seriously I did test my hearing, then had my ears professionally cleaned and retested my hearing. The difference was quite noticable on paper and in what I heard. And what comes out of your ears will suprise you.
I spent a lot of last week setting up demo systems with a dealer who has been at for quite a while. Two of his 'no-cost' tweaks that made the difference between decent sound and a system that sings (and sells) are:
He calls it 'absolute phase' (he is also a guitar playing, studio and live engineer) and notes that many pieces of gear can be reversing the absolute phase. In his store you will find many of the amps with the speaker connection's negative and positive reversed, still in phase between left and right of course. What he explained and demonstrated to me so that I could hear it, was that speaker's soundstage should be coming forward from the speakers, as well as back. When out of absolute phase the soundstage is all behind the speakers. This made much sence to me as I know that JBL Pro monitors are out of phase, which is easy to test, by putting a small battery to a speaker and seeing which direction the woofer travels, it should travel forward with a positive to to postive connection and if JBL can't get it right on a speakers, then it makes sense to believe other manufacturers may be missing this one too(including my gear, checks have been instigated). When in absolute phase you can then find you don't need to toe in your speakers so much when doing free tweak number 2:
Speaker toe and soundstage, start with the speakers straight out and ignore the vocals, which are generally centered in the mix anyways, first check to see if the bass is centred (a good live mix will usually have the bass down the centre, except orchestra of course), get that centred by toeing in if neccessary, then listen to the soundstage of the instrumentation and see of there is a whole in the middle and toe in until the instrument soundstage just meets in the centre. The more you can keep the speakers pointing straight ahead, the more you can make your speakers dissapear and the better the soundstaging.