dbx 166, 166A, and 166XL: See this article for an explanation of dbx's product-numbering system. All three of these are built on the same core schematic, essentially the same circuit, just with different controls and different specific components used. Originally I had only reviewed the "A" version because it seemed at the time (some years ago) that the A was the one most commonly found on the used market. But the XL is prevalent now, and there is some current interest in the "vintage mojo" of the original 166. The chronological order goes: 166, 166A, 166XL, and lastly 166XS. The XS is the new/current model, and dbx has told me the only difference between the XS and the XL is cosmetics and packaging.
As with most other old/new gear discussions, people always say the oldest ones sound better. The main point of comparing these three models is to find out whether that's true in this case. So to get to the point: I do think the older two models can sound better than the later 166XL--but only at some settings, in some ways. It's not a night-and-day difference, and at some settings you might not be able to hear a difference at all. Between the older two, it's even closer--sometimes I could swear I heard a difference, and other times I thought they sounded the same. Maybe it comes down to different settings, or maybe it's just placebo effect. Very hard to tell.
One important consideration in this test is that the knobs and the markings around them do not necessarily match up from one unit to the next. There are many variables in the potentiometer taper, the installation of the pots and knobs, and the markings on the face, that can stump any attempts to set two units identically just by using the knob positions. For example, when I set the 166 and the XL to "identical" knob settings, they sounded very different--and the 166 sounded way, way better than the XL! But when I regulated the input signal more carefully, and used the gain-reduction LED's to set the strength of the compression, the two units sounded almost identical (at moderate compression settings).
However, when I set them both to compress more strongly, the old 166 was a clear winner. It sounded much fuller and more "open" when compressing strongly, while the XL sounded more dull and choked. Again, the 166A was closer to the old 166 than to the XL, in this regard. It's true of all dbx units that they dull the highs when compressing heavily, but I'd say the older one does that much less than the new one.
But before you go rushing out to buy an old 166, remember that these units are often noisy; they used opamps that were not as good as what's available today, and the components have deteriorated with age. The 166 I have is somewhat noisy, especially on one of the channels, yet the A and XL are noiseless. So on the one hand, you could get fantastic results by sending an old 166 in to a tech (like AudioUpgrades) to have its opamps and capacitors replaced, and maybe see if there is a power-decoupling mod for lower noise, as there is with the older 160/161. But on the other hand you could expect those upgrades to cost over $200, so you would have to balance the total cost (perhaps up to $400) of an upgraded 166 against the cost of a used Symetrix 501 or Aphex 651, or a new FEA, Effectrode, or Markbass pedal, and decide whether the 166 is worth the trouble. Personally I don't think so, unless you can get the 166 for very cheap, or if you already have one.
For this reason, I recommend the model A as the one to buy. Potentially better sound than the XL, and potentially lower noise than the old 166. I have to say "potentially" because there are so many variables that can affect these results, and as I mentioned before, there are some settings where these units all sound pretty much the same. To be very clear, I am NOT saying the XL is "bad"... just that the A may be a little better.
All three models have been made both in the USA and in China. With the original old 166's, believe it or not, the Chinese ones are more ruggedly-built (and heavier). With the later A and XL models, the only way to tell the difference is by looking at the words printed on the back panel. The 166 I have was made in China, and the 166A and 166XL I have were both made in the US.
For all three, this is a one-space-high rackmount unit, with two channels that can be run in stereo or separately as dual-mono. It also has extra features such as gating, expansion, and peak limiting. It only operates at line level, it does not work well if you plug a guitar or bass directly into it. The 166 has only 1/4" in/outputs, while the A and XL have both 1/4" and XLR. The 166 has an attached AC power cord, while the A and XL take an IEC standard detachable AC cord. The 166 is 8" deep front-to-back, while the A and XL are 6" deep. The 166 and the A were made with through-hole components, while the XL is made with SMT (miniature) components. The 166 and A are built to take the abuse of steady gigging; the XL is not quite as rugged (but it's still decent).
All of them feature a "Peakstop Limiter", but unfortunately it's a terrible peak limiter. It works great as a distortion effect, though--seriously! As a bass distortion the limiter sounds really good.
The controls for each channel of the old 166 include: threshold knob and fast/slow switch for the gate; threshold and ratio knobs for the compressor; threshold knob for the limiter; a switch to engage the sidechain control input; output gain knob, and bypass switch. There's one switch in between the two sides for linking them in stereo or separating them for dual mono operation.
The A and XL models have threshold, release time, and a sidechain switch for the gate; threshold, ratio, attack, and release knobs; threshold for the limiter; and output gain and bypass switch. They also have three more switches:
"Over Easy" engages an automatic ratio control that changes in response to your signal amplitude peaks;
"Auto" engages automatically changing attack and release times; and
"Contour" reduces the amount that frequencies below 150 Hz affect the level-sensor that controls the compressor. In other words, with Contour engaged, the whole signal isn't compressed as hard when you hit high-strength low notes. This reduces the common effect of a compressor "swallowing" your highs during those low-note peaks. It can sound equally good (or bad) either way, depending on your usage and tastes, so not everyone will want to use the Contour switch. It's a good feature to have available though.
The tone overall is quite clean, and fairly transparent aside from the dip in highs during heavy compression. The A and XL are also quite versatile, with a wide usable range of tweakability to dial in exactly what you need. Of course the flip side of all the controls available is that it takes some effort to find the right settings; the oldest one is easier to dial in.
During my original review sessions I directly A/B compared the 166A to the 160A, with close-to-identical input signals, and I actually preferred the 166A; I felt that it had slightly better tone, plus I like the Contour function (which the 160A doesn't have). With every dbx compressor I've tried, I was able to dial in some very similar sounds/results--so they may have their small differences, but overall they all sound "like a dbx".
All three models are a very good value for the money with their punchy clean sound, low noise (in the more recent ones), ruggedness, and versatility--just don't pay too much for them. Even the A model is pretty old by now, and may need refurbishing in some cases. Whenever possible, try before you buy, and pay close attention to what Ebay sellers do and don't say about the sound and functionality of the unit they are selling.