Question

Which one would go first in the chain?

Answer 1

You could do this, however it would not be a true representation of the ADL 700 signal flow. In your chain you would want the preamp stage of the ADL 600 first. Then go Line Out of the ADL600 to the Line Input of the RC500. This would prevent double pre-amping as the line input of the RC500 bypasses the pre-amp stage. Now you will engage the "Line In" switch of the RC500. At this point you may utilize the compressor and eq sections of the RC500.

Keep in mind, this configuration will be different than using a true ADL700. For one, the compressor and EQ sections are slightly different between the RC500 and ADL700.

Answer 2

In essense, I believe what you are looking to do is create some sort of function where you are using a compressor or EQ on the actual channel strip.  I would technically advise using other outboard gear to do this function.  Putting two preamps together complicates signal chain flow, and also can create issues with increased noise floor, which would be undesirable.