I would try using the "built in" volume controls in the apps (Spotify, iTunes etc.) before buying something like that.
Some gain software will cause audible bit reduction at low levels.
On the other hand, that is cheap enough to just give it a try.
If I can, I always prefer analog gain controls. If poorly done, it can be noisy (noise floor) but for the most part, digital audio has such a large s/n ratio that it would be very hard to notice.
Mackie and T.C. Electric both make a fairly cheap analog volume knob (XLR).
https://www.tcelectronic.com/Categories/Tcelectronic/Computer-Audio/Accessories/LEVEL-PILOT/p/P0D71#googtrans(en|en)
Griffen made a nice looking digital knob but it maybe out of production?
https://www.amazon.com/Griffin-PowerMate-Multimedia-Control-Computers/dp/B01F63TOAC/ref=sr_1_2?ie=UTF8&qid=1545505802&sr=8-2&keywords=griffin+volume+knob
Me, I generally use the analog volume knob on my Amarra 4 (thanks Al!!!!) or built in app volume or a combination of the two (depending on current laziness level).