The Heritage Audio 609A two channel compressor is a dual mono or stereo linkable 2RU diode bridge style compressor / limiter with Carnhill transformers both in and out (6 total!) and some updated features like an all Class A signal path as well as a fast attack mode. Simply put the 609A is among THE compressors to get if you really want to hear what high quality analog can do for your tone, as well as controlling you dynamic range both during and after the recording process.
With a predictable action of both attack and release, it's easy to dial in. Also, bass heavy material with a fast release is not an issue with the 609A like it can be with some designs. You can choose the FAST attack mode for things like drums, but it also comes in handy if you have a vocalist who zips up and down the dynamic range and needs some extra control. There are plenty of release modes, and the A1 and A2 auto-release modes are just as easy as you would expect (A2 has a longer program release) which also means far less setup time on 99% of all sources.
The TONE of this unit whether compressing or not is instant 1970's flavor. It has a crisp sound even under heavy compression and limiting, vocals have that presence and pop without sound small or dark when pounded. Mix buss comes alive with a brilliant character that is hard to describe, but recognizable.
Please note the stereo link does not make the left channel the master control but rather it means they both references the same control signal for more even compression and limiting. In other words you would generally need to set the controls the same (especially threshold) on both channels for stereo work. You get used to it, it's old school.
Heritage's HA-609A is an easy choice for tracking (vocals and bass are normally what I compress on the way in, and it does dual mono) as well as final mixdown duties.