USB Length, jitter and Volume Control Questions

I've been working on a PC audio system for a few months now and I have a few questions that I cannot find the answers to.

I have a new iMac running USB>DacMagic DAC>Onix Sp3>Merlin TSM speakers. I hav had a Valab NOS DAC for a bit and have a Stello DA100 on the way. My questions is how does the volume work in these setups? With the DacMagic I can control the volume through itunes but not through the system menu bar. With the Valab I could control both iTunes and system volume. Is there a difference between the two? Does using the volume control in iTunes or system conpromise the sound? If I'm getting gain from the computer (regardless of system volume or iTuens volume) am I not essentially using the computer as a lowgrade "preamp".

Also, how long can a rung of USB cable be before the signal begins to degrade? Does USB transfer introduce jitter?

Thanks,