I have a pair of EAR660s and Retro 176s.
Very different animals. RB and I did numerous tests with his SH compressor too (he also has an EAR660 and RI176).
The 176 has a kinda-old-school thick tubey sound. Its color is a bit of a smiley face EQ and its my first choice with sounds that have annoying mid range peaks. I'm in the middle of mixing an EP in the vein of Norah Jones and the 2 Retro 176's in the buss were the best out of everything else I have... record to 1/2" at 15ips at the end and you're golden
The EAR660s are very different. Color wise they accentuate the high mids a bit but because of the low-mid bump and the smooth rounding off of the extreme high end (I'm describing how I hear things) they sound like the most beautiful 3D honey coating ever made. The EAR660s would NOT be my choice in a high-mid heavy vocal or arrangement. Having said that if a good engineer tracked the project you wouldn't have high-mid heavy tracks anyway.
Compression-wise, the Retro 176s are very smooth and give you the in-built sidechain filter which can prove a godsend sometimes. The Retro cannot be pushed to pump because of the nature of the compression and the slow release even at its fastest setting.
The EAR660's have the knee control which allows even more invisible compression than the 176s but also a fast release preset (1) which can turn the box into a drums pumper that will shame even the famous chandler TG1!
Based on the compression characteristic I would say that although it lacks attack and release controls, filters and output knobs, the EAR660 IS more versatile than the Retros. The colors though are so opposite that I would never think of using them in the same situation - they are definitely NOT interchangeable.
The Shadow Hills Mastering compressor is a beautiful comp. The opto section is very smooth and nice but IMO the discrete one is where it is. Its like an SSL comp with great smack, smooth at times and crush when you need it! Extremely flexible compressor (well...its like 2 comps in 1!).
Color-wise even with the various transformers it is not a "Tone Box" like the EAR660 or the Retro 176. The best way to describe it is that I would patch the EAR660 and RI176 in the path just for the color without using compression. The SH comp not as much. But if I think of a rock/pop comp that I need to push on the buss, the SH is king.
Hope this helped
