Aphex 320A Compellor: This is a two-channel rackmount unit, and it's quite different from the other comps I've reviewed. It combines compression, limiting, leveling, and gating, and those functions are all controlled in an interconnected way by an automatic "program adaptive" system.
It has only balanced XLR in/outputs, and will not work with your guitar or bass plugged straight in. The 320D model also has digital in/outputs via AES XLR jacks. I think the A version is discontinued, only available used.
The controls on the front panel do not directly set the parameters of the compression or leveling, but rather they set up a framework around which the automatic system makes its decisions. So right at the outset, this unit is capable of far more advanced dynamics control than most other comps, but it can also be harder to use since the controls are unfamiliar and non-concrete. It doesn't generate any noise of its own, although like all compressors it can amplify existing noise at higher settings. There is no rolloff of the lows, and the highs only roll off a bit at the highest compression settings. The tone is completely transparent.
The stated purpose of this processor is to normalize (match the output levels of) low-level and high-level incoming signals. So it's ideally suited to broadcast work. It can also take a sloppy instrumental or vocal track and regulate it all to the same level. I used it with two basses: a passive with fairly even output from string to string, and an active with an overpowering B string. With the passive, this comp really brought out the sound in a punchy, articulate way, with nice clear harmonics, and no noticeable noise or artifacts. With the active, the automatic levelling functions were trying so hard to regulate the signal that the noise level would jump up and down, and playing that B string hard caused a lot of distortion. Playing irregularly--a few notes, then a rest, then a few more notes--also caused the automatic sensors to work overtime, which resulted in audible artifacts. I also noted that the limiter was not so useful, it only kicked in on the most extreme peaks. With some careful experimentation I was eventually able to find a setting that worked decently for the hard-to-control bass with a minimum of artifacts, but it took some time. Whereas with the more even-output bass, with steadier playing, it was fairly easy to find a clean, natural-sounding, noiseless setting. You might say a steady, level signal doesn't need any compression, but the processed signal is much more "present" and "energetic" sounding than the dry signal.
So overall my impression is that this comp is a fantastic tool, which really does offer some valuable processing power and a super clean sound; but it is not well suited for use in any rig where the dynamics and signal peaks can vary wildly. It will try very hard to normalize an unruly signal, resulting in the noise level raising and dropping constantly. "Extreme" input signals can cause distortion or other artifacts. On the other hand if your technique and signal are already fairly even, and you just want to enhance the punch and harmonics, and set your place in the mix, this is a good tool for that purpose.
Price in USD: the 320A is $150 to $350 used, no longer available new. The 320D is $400 to $700 used, and $1299 new.