Go down and check coupons out. Remember to check price history if it is available. You probably want to know what is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon and price update (UTC):
Channel: 2.1, Bluetooth Version: 5.0(no shielding, 4-8Ω, Color: Black, 10m), Material: electronic components, Input sensitivity: 500mV, Audio Input: AUX+Bluetooth, Power: DC12-24V/5A, Model: ZK-TB21, Chip: TPA3116D2(with AM anti-interference function), Adapter speaker: 20-100W
Banggood
Banggood warehouse is located in China. I don’t think there is an active coupon right now – at least in my database.
Link: Banggood product page
Price: $-3
It looks like this item is out of stock in Banggood. If you found it then please send me the link. I will replace it.
Click here to search on Banggood!
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGTB21, BGGeeK13, BGTB211, BGTB212, BGRUMB15, BGGEE13, BG13geek, BGHONEY2110, BG47f234, BG64efea, 13f142, BG737caf, BG3552f1, BG494704, BG086e4d, BG42839e, 00c906, BG00c906, ADMITAD2212, 2022ACTIESNL, Terence, 8Couponsde, MVC8SPRING, 8BGwbg2204, 8wEbgaInS5, pROMo8cuPoN, hoNEYbGdeAlS, BGJUNEGSG9, Cuponation9, 9OFFSUMMER, BG8OFFCOUPON
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for ZK-TB21 TPA3116D2 Bluetooth 5.0 Subwoofer Amplifier Board.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).