Protocol on mixing -10db and +4db line levels

I'm putting together a bi-amped system and I'm planning on powering all four channels from a Carver AV-64 amp from the day. The Carver has a built-in 75hz crossover which I plan to try, but it may not be the ideal roll-off point for my speakers so I want to keep my options open. I happen to have an ART cx310 active crossover network that seems like it would do the job well, but given that it only has XLR and 1/4" I/O (definitely pro gear) and the Carver only has RCA (definitely home gear), I worry that sending what is probably a +4db signal into jacks expecting a -10db level might cause problems. But I'm not really sure how line voltages work and I would love some clarification. My whole system is going to be something like:

Computer -> USB DAC -> RCA cables -> Passive Attenuator -> 1/4" cables -> Crossover (either ART CX310 or built-in on Carver) -> Carver AV-64 -> Maggie MMGs and 2 Passive subs

Thoughts? Somebody want to explain why there are two different line level standards and why it matters?

Thanks!