Direct connection between DAC & Amplifier

Arthur Salvatore (www.highend-audio.com) recommends that amplifier be connected directly to CDP with volume control bypassing any preamp for the shortest signal path and therefore best sonic quality. I connect a USB DAC to a tubed amp without volume control, and have to use itunes' volume control. Will this digital volume control compromise the sound quality? Is there a better alternative to having an analogue volume control other than a passive linestage or TVC which requires interconnect in the signal path?