How to Add Remote Controlled Volume to Dac + Amp That Doesn't Support It

Not sure how much you want to spend, but a used Bel Canto DAC would do very nicely and not take up much space.  That’s what I’d do.

 https://www.audiogon.com/listings/lisai340-bel-canto-design-dac-3-24-192-d-a-converter-with-dac-3-7-digital-input-board-da-converters

Placette Remote Volume Control (RVC).  Essentially a one input passive preamp using high quality Vishay resistors.  Very transparent, meaning it doesn't add or subtract from the source sound.  They can be found used around $400-500.  There's also a 3 input version called Placette Passive Linestage.

Second soix' recommendation of a Bel Canto DAC. I have a DAC 3 that works perfectly as both DAC and remote for a tube integrated without remote. I think I paid $700 for it used a few year ago.