Here's my two cents, as I am interested in having quality EQ in my rig, and I thought, 'why not try digital first?' So, I just checked out out Hear, Boom, and DPS. They are all junk, in my opinion.
I am streaming Spotify using OS X Mountain Lion AIrplay (which sounds better than Airfoil, which I was using before, by the way) to an Apple TV, which I have connected via Toslink cable to a high quality DAC/ tube preamp, which feeds a good solid state power amp, which in turn powers very high quality (I hate the term 'audiophile') speakers that produce excellent frequency response from 30 Hz - 20 kHz. All this to say that I hear every little change in the signal, and the quality of the signal using no eq plugin is pretty good. Could be better I'm sure, but hey.
Point is, with these EQ plugins, the signal is audibly degradedeven with no EQ applied, simply with the plugins active. (Boom and the AU/Soundflower trick don't apply here in my case, as neither seem to work with Airplay).
I did this myself tonight, and thought there should be an easy to follow tutorial video for this sort of thing... but I didn't find one. So I made one. Hopefully this helps some people out (best viewed in 1080p at full screen size):
Thanks monking for the hint. Audio Hijack has been a great solution for me. I use the graphic EQ and/or the parametric equaliser (at around ~5KHz to remove some annoying sibilance I get on my system) depending on the recording. While it is true that it just gives access to the Apple built-in Audio Units, it is low hassle and has capacity for other/multiple plug-ins like VST and 4FX and many other things besides. But is does cost money...