Can you post a picture of the connections on the back of your subwoofer? Some subwoofers will have input and output connections with the intention of running audio to the sub first, then from the sub to the speakers/speaker amplifier.
Hmm, have you tried driving those [passive] speakers with your preamp? Looking at the product page, I’m not exactly sure what the douk will do, but typically a preamp would only have enough power to supply headphones or active speakers. Is your subwoofer active (powered)? When working with passive systems you need to pay attention to overall power/wattage delivered at what impedance/resistance, and what the speaker is designed to handle/require.
Next, do you have a cabinet of some sort for the sub? You’ll need one if you want any bass out of it.
Second, I would actually suggest a 2.1 amplifier to power everything. What you have are passive speakers and a passive subwoofer, meaning none of them have built in amplifier. Like @astra said, pre-amps don’t have anywhere near enough power for speakers/subwoofers, so you’ll need something extra.
That second item you linked in the OP looks like it would power your speakers (via the two sets of binding posts), but not the subwoofer. I’d recommend looking for something like a Lepai LP-168HA or similar. Searching “Douk 2.1 amplifier” on Aliexpress seems to be returning stuff that would work.
To better understand what you are looking at, it would help to know where the music is coming from (as in: Computer with 1/8 jack and optical, phone via bluetooth, TV with SPDIF and optical, etc).
Like @astra mentioned, you need to pay attention to the power ratings.
The sub is 4 Ohm, 250W RMS, 800W peak
I take the speakers do not have a watt or impedance rating on them?
If you have a multimeter on hand, hold that accross the wires coming out the speakers. Should be somewhere between 2 and 8 Ohms (± 2 Ohm)
From the pictures of the speaker and sub, you need an amplifier (or two). One for the speakers, one for the subwoofer.
One easy option would be the SMSL AD18 for the speakers and feeding the amplifier for the subwoofer from the sub-out.
Make sure to get the version with the port marked “coax opti1 SW” as shown in this image:
More advanced ones will have a SPDIF input for LFE, but then you’re getting into multi channel setups.
To the topic at hand, to do what you want with what, you have you’ll need a separate amp, with cross over for the sub, and possibly a separate amp for the L/R speakers. This can be a PITA for various reasons, and can get expensive quickly. You’ll also need a box/tube for the sub, and boxes for the speakers if they are not already mounted/enclosed.
Unless you’re really into the electronics of making different things work together it is generally cheaper and easier to get a 2.1 system for a PC. The higher end ones are around $200 US. Admittedly the Audio Gear side of things can be fun.
If you know that 7.1 is your goal you can get a 7.1, or greater, AVR and run your signal out via HDMI and then the video back to any source with HDMI from any of the outs on the AVR. The system can be configured to do anything from stereo/2.1 to as much as 14.2, 9.4.2, depending on how much you’re willing to spend or how much you’re into audio. SPDIF/Optical also works, but you can run into problems when it comes to bitstreaming multichannel sound, in my experience.
A cheap but good 5.1 set of speakers, like from Polk Audio, is generally around $200 ish (you can obviously spend more or buy piece by piece which does add up it’s all preference) and includes a sub and all your satellite monitors; you’ll just need speaker wire and an AVR obvsiouly.
The biggest challenge of doing the AVR system setup is learning how to config media players to bitstream multi channel sources.
Do more research ! Something things are way easier than that and other things are far more complicated than you can imagine. Politics , DRM, bit rate and bit depth. There exists in this day and age 2.1 systems that can do a fairly decent job of giving you a much fuller surround like sound stage. Even give you depth…
Investing in a powered subwoofer can and does save effort and headaches.