Code/model numbers for dbx products: People easily get confused by dbx's product numbers and suffix letters, and it's hard to find information about them on the dbx website. For their more modern units, the general rule of thumb is that they changed the model number whenever they changed the circuit design, and they changed the suffix letters whenever they changed to using a different factory, assembly technology, or in/output design, without changing the main circuit design.
The exceptions to this system are their oldest units with the fake wood-panel sides, like the 118, 119, and original 160. Sometimes the 160 (with no suffix) is unofficially called the "160 VU" because of its large VU meter, and to distinguish it from the newer ones. The 162 was a stereo version of the 160, and the 165 was a 160 with more controls over the compression. The 161 is the same as the 160 but with unbalanced RCA jacks instead of the 160's terminal strips and transformer-balanced output; the 118 is the same as the 161 but without a VU meter. Those two are popular choices because they can be bought for less money than a 160 and then modded with transformer-balanced outputs to get the same performance as a 160. The 117 and 119 are other variations on the same basic design, but they are less popular because they require more modding of the controls and circuit to get good results.
In all of those cases, including the coveted original 160, it is a really good idea to have a competent tech go through and replace the electrolytic caps and any other dying old components. There are also other upgrades, such as the "decoupling mod" for lowering the noise floor; or the "Burr Brown" mod, which means installing better opamps, regardless of whether you use opamps made by the now-defunct Burr Brown company.
The 160 had discrete hand-matched transistors and mostly point-to-point wiring; they redesigned it with printed circuitboards, using IC chips for the VCA components, and renamed it the 160X. The X version could be bought with or without transformers on the outputs, and transformers could be installed as an aftermarket upgrade--so when buying a 160X now, you may or may not find an output transformer inside. It had barrier-strip in/outputs. Then they made a version with XLR and 1/4" jacks instead, that had two outputs: one with a transformer and one without; and called it the 160XT. Somewhere in that period of time they moved production from Massachussetts to California, Utah, Japan, China, Hong Kong, and Korea. They also went from assembling the units by hand, to robotic assembly; and from through-hole components to SMT (miniature) components. At that last stage they renamed it the 160A.
The 163 was designed to be much cheaper than the 160X, though it had hand-matched transistors like the 160; later it became the 163A when they switched to IC chips, and then 163X when they switched to SMT components.
The 166 was created as a two-channel version of the 163A, with additional features and controls. The 166 later became the 166A with a few changes to the controls and the manufacturing. The 266A was a cheaper (and lower-quality) version of the 166A, with no "Peakstop" limiter. Those became the 166XL and 266XL when they went to being made with SMT components. The 1066 was designed to be a step up (in quality and price) from the 166XL. As of 2012 the newest suffix for the 166 and 266 lines is "XS", and dbx has informed me that the only difference between XS and XL is cosmetics and packaging.
Of course they have many other models, but hopefully the information here will help you sort through the more common and confusing number/names. Also, many of dbx's records of their past production are lost, or only partially known. I have compiled the info here from statements by dbx engineers and related industry pros, but even they disagree with each other about some of the details, and they also admit there's a lot that they simply don't know anymore.