This is a wireless transmitter designed for powered speakers. You don't have powered speakers.
You need an amplifier and volume adjustment for your speakers. The speakers you have are not internally powered (your subwoofer is, and that is normal, but your speakers are not)
You are very excited to get this working, but it is clear that you are new to audio/video. Slow down, spend some time reading and understand, that Craigslist and other options listed are really the best and least expensive ways to get things done. The guys on this forum have been doing audiovisual work for decades.
I'm not following what you are doing with your laptop, and if you have a Xfinity cable box, then that is currently wired into your TV with a HDMI cable right? And when you hook your laptop up, you are hooking that up to the TV aren't you? This means that the TV you currently own is handling all duties for both audio and video. It means you are using the jenky speakers built into the TV instead of the better speakers you have and the subwoofer.
The point of the AV receiver is that it handles everything at the best possible quality. But, if you want to keep the video going to your TV and your TV has a digital optical audio output, then that's a decent way to get audio out of the TV and into a used AV receiver that doesn't have HDMI, but does have optical audio input on it.
The AV receiver provides the power for the speakers you have. That's CRITICAL!!!! Most proper home theater speakers are not powered. They are designed to connect to an external amplifier, typically a stereo or AV receiver of some sort.
SOURCE->AV RECEIVER->SPEAKER WIRE->SPEAKERS
SOURCE->AV RECEIVER->RCA 75 Ohm CABLE->SUBWOOFER
This is the standard in the industry.
The previous link you provided, for the stereo amplifier, isn't the worst way to go and will work, but I would MUCH rather see you get a AV receiver off of Craigslist as you are sure to be able to get something locally that will be MUCH better. People get rid of old receivers all the time when they upgrade. The claims of 300+ watts from that little box is also a blatant lie for actual power delivery.
FOR EXAMPLE:
$40 -
Home theater receiver 5.1 Denon AVR-591 No speakers Remote included Perfect condition Pick up only
philadelphia.craigslist.org
This one is my favorite of what I went through quickly. It has both a digital optical input, which may get surround sound out of the TV, but it also has HDMI inputs so you could connect sources right to it for the best possible audio from those devices.
$100-
Denon A versatile A/V surround receiver with a 7.1-channel amplifier, the Denon AVR-1611 is designed to be the centerpiece of an exciting home theater system. It supports Dolby TrueHD and DTS-HD...
philadelphia.craigslist.org
Bought this to use as a phono amp only to find out I already had one. It was purchased used, but it's in great condition.
philadelphia.craigslist.org
For sale is a Denon AVR-1613 audio-video receiver. Purchased c.2014. Units is in great shape & works fine, I just decided to upgrade. Specs:...
philadelphia.craigslist.org
These are good, but not as good of a deal as the one above for $40.
The rest of this stuff really needs to use the optical output from your TV, assuming your TV has an optical audio output...
Denon receiver in perfect condition and everything works.
philadelphia.craigslist.org
Yamaha receiver in Excellent condition. Great for home theatre or sound system
philadelphia.craigslist.org
Excellent condition Yamaha RX-V1200 Audio/Video receiver 6.1 channel 80 watt Still available if posted I can meet locally No remote ,only power cord included
philadelphia.craigslist.org
There are quite a few good products out there which have decent connectivity, offer a great deal of expansion, and more. They may not have integrated Bluetooth reception, but if that's important, Bluetooth receivers can be had on Amazon for 20 bucks to add that functionality. Keep in mind, wired audio is ALWAYS a better way to do things. It's a lesson to learn now... WIRED IS BETTER! In almost all circumstances, while it isn't always as convenient, it is often more reliable, almost always better quality, and more consistent. Networking, audio, video... wired if at all possible.