I wanted to improve my signal chain a couple of years ago and decided to go the 500 series route and ive been very happy. I noticed a massive improvement with the audio i was able to record into my apogee interface.
Ive had a pair of Rupert neve 543 for a while that i absolutely love. They are the cleanest compressors ive come across. The newest compressor i have is the Docderr by ELI. Its an amazing compressor/EQ/Saturation unit. Its similar to a Distressor in its saturation and comp but it can be used as just an EQ. As far as preamps go i went for the MONO Gama, TG500, and 73 JR by Heritage. I use the Gama on vocals, synths guitars, basically on anything i want clean but to have girth. I use the 73 on audio i want to have an little more of an edge and to cut through. I use the 73 almost always on electric Guitar. I use the TG500 on sources that i want to have an edge and cut through but warmer and tighter than the 73. Ive also gotten to use a pair of Inward Connections Brutes, and those were spectacular on vocals and guitars esp electric guitars. As ive played more with these beasts ive explored many different uses and combinations for sounds but these are all my starting points.
Id love to try out some API gear next esp the 512 and 525 or 527