I do this all the time using the I/O plug in (a logic plug in that is) and the
Expert Sleepers Latency Fixer. Works like a charm. BTW I have never worked with the ensemble or maestro.
Make a copy of the track you want to insert the compressor on, just for checking what latency you have. My method is to do it by ear, I´m sure there are other methods that take much more time
(you only have to do this once, unless you work in different sample rates).
Just connect the compressor/whatever unit to an analog I/O of your interface and send the signal through it with the I/O plug, prefferably without any gain reduction, so you can check your latency.
Solo both tracks in the arrange window... You MUST have the CPU saving (or whatever its called) solo mode and push the solo in the arrange window, not in the channelstrip to the left. If you solo your channelstrip, you will also have to solo the output that the I/O plug is using...
Make sure you´re playing them at the same level. Then insert the latency fixer plug in on one of the tracks, select that it works in samples in the plug in menu. Then just dial in the latency with the sample fader, you´ll have to listen very carefully to the phase shifting once you come close to perfect match, there´s only one value that is the correct one.
Maybe you have some documentation that came with your converters that tell the latency and will make it easier for you. I just listened and got it right. My latency is 213 samples @ 44,1 kHz, I´m using motu HD192 interfaces.
The thing is, it will never be perfect, as the smallest amount of compensation you can do in a DAW is one sample. You work in 44,1, the latency will always be between samples. I have no problem with this as it is the only way to insert hardware into the digital world. I´m even doing parallell compression with this method and it sounds very good with a good compressor. So I´m all happy...