jw944ts, When cable impedance is perfectly matched to DAC then length of the cable doesn't matter, but when it is mismatched then you'll get reflection from the end of the cable. This reflection originates from the beginning of the transition (knee) and might come back exactly at the middle of it, where level change is recognized, possibly shifting it in time. We can avoid it by delaying reflected signal using longer cable. For typical dielectric electricity moves at about 2/3 of the light speed - about 5ns/m. With 1.5m cable reflection will come back at 3m*5ns/m=15ns just missing half of typical 25ns transition. When transition is longer, for instance 30ns than you need 2m cable, but when it is shorter than 1m might be enough. Hard to predict and almost impossible to measure, so the best way is to try. 2m sounds like the safest choice, but it is a little more prone to electric ambient noise. For DAC3 I would use USB, but again the best is to try.
Optical has slow transitions making point of level recognition (threshold) unstable in presence of system noise on either end. The benefit, of course, is electric ambient noise immunity and no ground loops. I use inexpensive glass Toslink having both ends (DAC3 and Airport Express) plugged into conditioner (don't have USB source). In addition DAC3 has strong jitter suppression. Hope it helps.
