The Dbx 161 is a vintage, single channel compressor/limiter that is no longer being made. The unit is made up of analog components, and I believe it has XLR connections but I'm not 100% sure as the one that I've used was already racked and ready to go in the studio I used it in. It seems as if you can get this both as a stereo pair that is rack mountable, or a single channel unit that will take up half of two rack spaces.
I didn't have any trouble using the Dbx 161, but how well you catch onto it solely depends on how much experience you have with other compressors. Having said this, the 161 does have a pretty basic make up and as long as you understand the basic parameters and components of a compressor, you shouldn't have a problem with the 161. It simply has knobs for threshold, compression, and output gain. It also has a VU meter which can be programmed to show either input, output, or gain change (reduction). A manual isn't necessary for this, as if you don't know what these parameters are a good place to start would be by doing some research on compressors outside of the 161.
Having used a few other units in the family of the Dbx 161 like the 165 A, I think it is safe to say that the 161 falls right in line with them in terms of sound quality. It has a pretty wide sound for a compressor, and in my opinion is great for use with all sorts of drums. I haven't yet had the chance to use it outside of using it on kick drum, snare drum, and floor tom, so I don't know how well it would work with other instruments or voices yet. I will say that it has a great vibe and was overall very pleased with the tight sounds I was able to achieve while using it on the aforementioned drums...
I haven't seen too many Dbx 161s out there, but I have been a few different studios that had them and said they used them often. The price of one of these definitely wouldn't be too expensive if you could find one used, but they definitely aren't all that readily available. In my opinion the 161 and this family of compressors has a much warmer and thicker sound than that of the newer Dbx units, which for me is a good thing. I think that I'm a bigger fan of the 165, but the 161 is a great unit as well that is definitely comparable...