EDIT: I wanted to clarify I’m really just speaking on hardware vs. software for this particular reference – bus compression. There’s other areas like EQ’s and Pre’s where I find certain hardware to have a much more substantial difference. Hope that helps.
I don’t have an SA4000, but I have a Tegeler Creme that includes what’s more a less a very similar G style compressor. I did compare it to the UAD along with several other software emulations (Waves, The Glue and a couple of others). I also have to keep things in context… I use it as a bus compressor largely for mastering, so I’m not ‘slamming’ it. I’m generally never more than 2-3db gain reduction.
At lower compression levels @ 2:1 or 4:1 ratios the UAD is very close to my hardware. Eerily so at times. You’re basically chasing the last 5% or less of difference most of the time. Where things start to change more is the harder you push the compressor and with more aggressive ratios. Then it starts to get a bit more apparent. When you’re really pushing it (levels no one would likely do except for some creative effect) the hardware maintains its composure much better than the plugins. The hardware never sounds ‘bad’… just too much compression whereas the plugins start to sound pretty ‘meh’ when pushed that hard. Overall, I prefered the UAD the most out of all the emulations, but that’s not to say any of them are bad. For my ears, the UAD sounded closest to my hardware emulation.
What gets overlooked a lot I think when people compare plugins vs. hardware is the make-up gain and just gain staging overall in the analog domain. That’s really where it’s more pleasing for me than just straight compression differences. Adding gain in software isn’t adding much to the overall sound (just gain) whereas adding 2-3db on the output of the Creme is also adding subtle flavor. I could try to give you some cliche analog platitude, but those are completely subjective and personal, so I think it’s kind of meaningless in the end. It just sounds better.
If I couldn’t have the hardware I’d be very happy with the UAD. The hardware is a luxury IMO at this point and I’m fortunate enough to do it, however, I don’t feel it’s a requirement for most people. I think in most cases you’re looking at 5-10% differences in varying elements and not always at once. It’s very situation dependent. Some might call that ‘night and day’, but I genuinely think its about overall context. If you want that last little bit for whatever your situation is… awesome.