Headphone leakage to main monitor

[Impala]

Hi,
I use NS10s as main monitor and PMCs as alts.

1. There's leakage on NS10s even Alt mon is selected. (I hear the sound deeply even the Alt volume is down) (I didn't notice any leakage problems with my PMCs)

2. I use my LCD monitor's built-in speakers as a low-level monitor by headphone out of C24. When I start to turn the hp volume up NS10s start popping (like a dead tweeter)

3. btw, any other comprehensive user's guide or sth like a webcommunity for C24 users?

thx