I use the Bedini Clarifier sometimes but honestly I'm not sure if it makes a difference. There is also a CD blacklight mat that lays on top of the CD that I don't really use anymore.
Another good tweak is running a demag track. Ayre makes a disc that is supposed to be really good.
The best tweak of all, however, for newer equipment is break in tracks. Glide tones for headphones and speakers and brown or pink noise for amps works best I think. This is the only "tweak" that I am totally convinced is real.