The API Audio 554 is a three band, semi-parametric equalizer. It is a 500 series (lunchbox) style module, only fitting within these confines. It is an analog piece of gear, consisting of no connections of it's own, except of course the connections on the lunchbox. This isn't rack mountable in any other way than a 500 series casing.
Anyone who has a bit of knowledge about parametric equalizers should be able to use the. API Audio 554 without any issues. It's really a simple equalizer, with three bands that each have tiered parameters for gain and frequency. Each band also has a bandwidth switch, for either a wider or narrower Q. I'm not sure how wide or narrow exactly each setting will give you, but it's noticeable upon listening. Lastly, it has a button for engaging the unit to affect your signal. I haven't seen the manual for the 554, so I can't speak to it's make up.
The sound of the API Audio 554 is totally clean. It's easy to make any EQ move within the range of these three bands, so this is usually the EQ I'll go to when I'm just looking to make a quick and easy change. We have a single 554 in one of our smaller studios on site where I work, and it definitely sees a good amount of use since it is in a room without a console. It definitely isn't the most versatile EQ out there, but again for simple equalization it's perfect. It's a very sterile EQ, as I don't find it to color my sound at all...
With the API 554, what you see is what you get - a really simple, but highly effective parametric equalizer. For whatever reason, I don't believe that they make the API Audio 554 any longer, but I don't think it would be too hard to get one used. The API 550B is definitely a more versatile equalizer, but in most situations I wouldn't think it would be worth the difference in price. If you've got a 500 series rack, the 554 is definitely an EQ that you should be aware of.