Yes. Optical out will let you use the DAC in a receiver that has optical inputs.
Small receivers? Difficult if impossible to find. Units like that Teach AH101 are pretty rare. Even the Marantz slimlines
have the same foot print as your average AVR. If you can bear the size of a full-size receiver, your options and choices open up greatly. For example, here's a brand new ONKYO TX-NR414
for $199 (promo code EMCXVWS49). Doesn't cost much to buy one then.
I can use the volume control on my keyboard with my USB DAC. Not certain about optical output.