Subwoofer question?

Both answers you have already are correct. Swapping out the speakers in a home theater kit isn't a great idea because they're designed as a complete system. They are built down to a price, so the amps inside the head unit aren't designed with the extra capacity to drive better speakers. They are only adequate for the speakers that are supplied. What LG refer to as a subwoofer is really just a bass speaker in a box. The system's amp isn't capable of producing deep enough bass to justify adding a better speaker; it just doesn't have the frequency range or the raw power. If you want to try it anyway then go ahead. You will get some sound from the speakers you swap out. What's going to be missing though is the extra volume and the deep rumble that you are hoping for. Those bigger speakers are going to be power hungry for electrical current that the amp can't provide. It'll be like hitching a really big trailer to a very small car. From an electrical point of view, speakers don't have a fixed resistance. That's why the word "impedance" is used. The amount of resistance varies with frequency. So the impedance is an averaged figure for resistance based on the reaction of the load to an alternating signal. A speaker with an impedance of 8 Ohms might go as low as 2 or 3 Ohms at some frequencies and as high as 30 Ohms at others. Starting with a 3 Ohm load then the resistance might drop well below 1 Ohm at certain frequencies. We talk about speakers as a load on an amp and it implies that we are "pushing" power in to a speaker. What's really happening though is that the speaker is sucking current from the amp. The lower the resistance then the more current the speaker sucks. If the amp can't keep up then it blows the output transistors. That's why a simple parallel resistance calculation isn't sufficient to model the behaviour of a speaker in a circuit. If you are still keen to try replacing the bass speaker, then try your 4 Ohm sub speaker on its own. That way you know that 100% of the available power is going to one speaker rather than just being wasted in the 12 Ohm sub that the amp can't drive so well.

You could but keep in mind the 4 ohm driver will be getting 75% of the power and the 12 ohm will be getting 25% of the power. mk