how to best add a volume control
If all you want is a simple pot or volume control here is a sorted list of the most popular products available worldwide.
So that is your immediate problem solved for as little as $30.
On the other hand, if you are looking to the future and have a little money to invest I would like to point you in the direction of an audio interface.
An audio interface is almost always the answer to any question related to active monitors. Active studio monitors and audio interfaces evolved together in the 1990s in response to the then new popularity of digital audio and the use of DAW (digital audio workstation) software in home or project studios. They are designed to complement one another perfectly. Initially this gear was very expensive and meant for professional use (think Genelec and RME) but there are now a whole host of well priced and versatile products suitable for domestic use.
So for as little as $50 more than a basic volume control you will get a whole host of additional features which you will almost certainly find very useful in the future. Such as:- A fully fledged monitor controller, DI box (for conversion between bal and unbal and isolation from earth loops), A headphone amp with separate volume control. The ability to connect virtually any new source - PC, tablet, phone, CD player, microphone, instrument, wireless receiver, games console anything at all, The ability to connect multiple outputs - separate zones or surrounds/subs, be able to cue and prelisten, monitor, meter and analyse inputs and outputs, apply EQ and Effect to any and all inputs and outputs. I could go on but I hope you get the general picture.
Of course you can do some of the above with an AVR or receiver as well but in a much bigger and ugly box and since you have already started down the road of active monitors the interface makes much more sense.
If you are interested here is a selection of what's currently on offer.
That's a pretty daunting list so I will mention the companies and products that are currently most often recomended.
At the entry level : Focusrite Scarlet range. Native Instruments Audio 2 and Audio 6, Steinberg UR range, Roland and the less expensive MOTU products .
Top of the range units are the RME Babyface, SPL Crimson, Audient ID22 and MOTU Ultralite.
Take a look.