How loud is your work environment, and how far away will you be from the stereo? A typical listening volume will hit peaks of about 85 dB, which your speakers will be able to produce at 40 feet away using the SMSL-SA50. If you're 5 feet away, it'll produce a deafening 103 dB. That also doesn't scale very well with extra power; you'll need a lot of power to make a small difference.
That Fosi amp is rated in a misleading way again. It uses two TPA3116D2 chips, which are good, but they're not really good for 100 watts. More like 80 watts clean (you should only look at power where Total Harmonic Distortion (THD) is <1%). But that's on a 4-ohm load, and your speakers are 8-ohm... So power will be half. And that's if they've supplied a 24v power supply capable of actually producing enough power - which, from the pictures, they haven't. Overall, it just doesn't look like a very high-quality package.
Honestly, speakers need a lot less power than people think, unless you're building a home theater system and aiming for (totally unnecessary) "THX Reference" levels, or are doing live sound reinforcement for a concert. But if you're really concerned, just get a home theater receiver from Goodwill and use that - it'll be good for a true, clean 85-125 watts and you won't have to worry about power supply sizing. If that's too bulky, you can at least use it for a while to reassure yourself that you're not demanding full power from this thing. Or you can just go nuts and get the big daddy SMSL SA-98E.
You'll definitely want to put some fluff in the box - probably cheap Polyfil fluff from Walmart, and you'll use a bit of trial and error to find the right amount. You normally stuff sealed boxes like this, and line ported boxes.