Is the volume control on the Transport or the DAC?

It seems a silly question but that's my ignorance. I am currently using an Audio Aero Capitole CD player in my system to drive a pair of monoblocks. I do not use a preamp nor am I contemplating buying one.

I am looking for a back up player and have spotted an Audio Aero Prima Mk II but the latter has a volume output of 2.2V, no volume control but has a digital output.

My question is, ifever the transport/disc reading of my Capitole becomes capricious, will I be able use the Prima as a transport and the Capitole as a DAC? If so, will I be able to control the volume level through the Capitole?

Or simply put, does the volume control of a Transport/DAC combo lie with the Transport of the DAC?

Second question, I am being offered a perfectly working Resolution Audio CD55 at a bargain price (around $800) which I am very tempted to buy as back up in place of the AA Prima but someone very knowlegeable in the field has recommended me to buy a used Resolution Audio Opus 21 instead. But times are hard and I am not keen on splashing out twice that sum on a back up player. Should I go for the Prima or the Resolution Audio. Both are at the same price point. I like the sound of my Capitole and play all kinds of music but more classical - Tchaikosky & Strauss type.

Many thanks for your advice and help.