Controlling studio monitor volume externally

I have a Panasonic Plasma (only audio out is SPDIF) connected to a fiio D3 DAC via the SPDIF. Attached to the RCA out on the fiio are two M-Audio BX5a monitors. I want to know the cheapest way to control the sound without having to adjust it from the back of the monitor, via a knob or remote or something.

The BX5a has two XLR ports on the back that I'm not using, if that helps.

Visual of how it is set up:



These speakers are connected via the TRS input to the left/right RCA out on this (fiio D3):



I use the optical in on this to connect to the optical out on my plasma. This all works so far, but I can't adjust the volume easily.



So if I purchased this (http://www.ebay.com/itm/BRAND-NEW-Axxess-Universal-Car-Amplifier-Bass-RCA-Level-Volume-Control-Knob-AALC-/271014397041?pt=US_Audio_Video_Remotes&hash=item3f19b78471) and two XLR to RCA cables, I'd be able to adjust the volume? I noticed the knob has 4 RCA cables (I'm guessing in and out)...would I need to plug the knob into the fiio as well as the speakers? I don't know how/ if I could do that.



Does anyone know if this will negatively affect the audio in any major way? Also, would I still be able to connect a sub with this setup, or is this the end of the line? Thank you!

I can't plug it in because I don't have it (the knob or the two XLR cables), obviously. I am considering ordering it, but I don't want to waste my money if it's a simple problem. Furthermore, it takes awhile for all of the components to arrive. It'll cost about $20 all told, and returns won't be easy.

I figure with this being an audio forum, maybe someone has the same problem that I do and can offer a solution. I've heard of using studio monitors on a television, but most are connected to receivers, which I don't have the money for.

Well I got the components in, and...the knob doesn't alter the volume much. Very, very small changes.

I want an external knob that can turn the volume down completely. Any ideas? Maybe I'm doing something wrong.

That's out of my price range, unfortunately. And furthermore, wouldn't that be redundant since the speakers have an amp built in?

I'm trying to do this as cheap as possible. Who would have thought that something as simple as changing the volume on a studio monitor would be so expensive! I might have to go the cheap DAC/Amp route, if I can find one.

EDIT: Ah, found the solution. Wasn't plugged in correctly. I have a mess of wires snaking around the back of my TV, but it works! Thanks.
Edited by Pojosama - 1/26/13 at 6:03pm