usb vs toslink?

i have a trivista DAC. or my krell 20i also accept digital input ( coaxial or toslink ), i am fairly happy with the sound of krell, or trivista, so my question is that if i get a hagerman and connect to either dac, would I expect a margiinal or noticeable improvement vs, apple AE to either DAC.
i have try apple AE , it's ok for background music, but not for serious listening

is usb to dac ie. hagman better than toslink from apple AE to DAC?

why the 15 ft limitation for usb cable??

thanks bruce,
i just order the hagman usb , i'll see how it go

hi bruce, i notice you mention you use a long coxial cable not i assume a shorter usb cable, why ? why not the other way around??