There are a number of options for surround sound output on a Mac:
Special hardware (like the USB "sound card" you linked to).
Connecting over HDMI to an A/V receiver.
Connecting a Mini-TOSLINK cable to your headphone jack.
There are pros and cons of all of these. The first is the most expensive option, and depending on the hardware you choose may not have any significant benefit over the other two. (If we're talking about the SoundBlaster hardware you mention above, the only potential advantage of that is that it has line level outputs you could connect to a set of cheap PC 5.1 speakers; there's little point using an optical port attached via USB rather than the one already built into the headphone jack on your Mac.)
The HDMI option is great, but it does take up the DisplayPort or HDMI socket on your Mac, and it can cause problems if you have a display attached but don't want your Mac using it. It should have the lowest latency, if that matters to you, and it should also allow the use of multichannel PCM (i.e. no lossy encoding) everywhere that it's supported.
The TOSLINK option works very well for DVDs and other sources where the audio has already been encoded using Dolby Digital or DTS. For other audio sources, you'll get stereo at up to 192kHz (depending on your receiver and on what you set your Mac to do). If you want surround audio from games or other software that can generate discrete channels, my company just released some software that enables that.
You can of course use a USB sound solution that supports macOS. However, that may not be necessary. What application would produce 5.1 sound? Can your speaker system deal with optical connections (TOSLINK)? – Daniel B – 2017-01-15T17:48:04.917
Oh yeah, so I need software too... forgot about that part. Does Creativ have any OSX compatible systems? If not, who does? – Kevin – 2017-01-15T17:49:49.167
Boom2 is never worth keeping - nasty & crashy:/ Just Google "mac compatible surround sound 5.1" for a swathe of options – Tetsujin – 2017-01-15T17:51:00.113
The company is called Creative. However, it really doesn’t matter what you buy when you don’t have applications outputting surround sound. Do you want to watch movies? Or perhaps play games? If you’re satisfied with Stereo, that’s fine too, but it’s relevant when answering the question. – Daniel B – 2017-01-15T17:52:59.707
Nah don't care about anything but music. iTunes, occassional Spotify. Want to make use of these speakers and enjoy my library. – Kevin – 2017-01-15T17:53:58.247
Music doesn't need 5.1 - almost nothing is mixed for 5.1 except some very specialist stuff, usually available on DVD. If all you need is stereo, then just use stereo. Add a sub if you want, but get one that can be run directly via the regular stereo out rather than a cinema system that requires a separate channel. – Tetsujin – 2017-01-15T17:56:01.293
1A lot of receivers also have an option to pump stereo out all speakers. Left speakers get left channel, ditto right, and sub and centers receive a mixed version. At least that's how my old Pioneer works. Just has a "surround stereo" mode. Most MBPs have toslink 3.5mm jacks, but in my experience it's almost never worth the effort unless you have a software stack that can use it. – peelman – 2017-01-15T18:03:32.120
Most on-line music does not have surround content, so the best you can do is connect using your existing stereo jack and use whatever surround processing is available in your sound system. An external sound card may give a quality improvement if you use the optical connector, but it won't give you more channels than are in the original source. So see what you can achieve with a stereo cable before you spend money. (Sorry for some duplication with @Tetsujin's comment, which appeared while I was in the middle of typing this). – AFH – 2017-01-15T18:04:14.147