Passive preamps: What are the relevant numbers?

I'm trying to decide whether I am going to add a passive preamp to my system, but I've been reading that system compatability is very important. What numbers should I be looking at in the specs of my CD player (Rotel 1072), Amp (DNA 0.5) and Interconnects to judge whether a passive preamp might work well in my system?

I know that there was previously a thread about this, but it seems to have been deleted.