If you’re going to use additional speakers like subs you’ll want to consider the number its outputs. The voltage of these outputs is a good indicator of its ability to deliver quality sound. 4 or 5 volt is above par. This seemingly small difference is big.
Adding a 4 ch amp will increase power to speakers. Many features are available. Shop around. As previously stated all those mainstream brands have good models. You get what you pay for is mostly true. A $99 boss will not be as good as a $999 alpine. The features, technology and warranties will vary widely. It’s overwhelming to determine what features you’ll appreciate most so shop, decide what appeals to you and go w that. Lots of choices, buy from a reputable seller to avoid headaches.
Amazon sells stuff that doesn’t exist in some manufacturers product lines. Meaning online sellers make available counterfeit products and bait and switch is common too.
“Oh, $100 cheaper?” Used, defective, missing parts, no box, many reasons to buy from legitimate sellers which are also on
Amazon. The big ones price match each other so it really doesn’t matter as long as you’re a conscientious shopper.