Every now and again I see questions asking about these inputs on subs, and there seems to be some confusion, so I thought I would post a thread explaining this that could perhaps be added to the FAQ section. I'll try answer this to the best of my knowledge. The basic answer to what these do, is essentially they do the same job, they just do it a slightly different way. They are both inputs, and they both take a signal from a source to produce the bass content in material. The main difference is that the low level input is designed to work with the LFE channel encoded into movie material you find on DVD's and BR's. The high level input is designed to work with the normal speaker channels, and is really left over from the days before processors and digital soundtracks with a .1 channel existed. For a more detailed explanation of what they do and how to use them, I'll explain them one at a time. First is the normal LFE input. This is typically seen on all modern processors and AVR's. The LFE channel is a discrete channel the carries the Low Frequency Effects added to movies. This is in addition to the standard speaker channels. Some processors have 2 sub outputs, but they both use the same singular LFE signal, so for the sake of this guide, we'll ignore this and talk of the LFE as a single sub channel. A typical LFE input on a subwoofer is a phono socket input, and his input will have its own crossover and gain controls. The gain is used to match the subwoofers output to the rest of the system. The crossover is used to select the crossover frequency at which the subwoofer will cross into the frequency range of the speakers. Pretty much all processors I know of also have these controls integrated into the processor, so the controls on the sub are somewhat not needed, but they are included anyway just in case. The main use for the LFE input is for use in a bass managed system, like a home theater with a dedicated subwoofer output. The high level connection does not use the low level phono connection. The high level connection instead takes a full range speaker signal. Typically this is done with a cable that connects to the front left and right speakers. This cable reads the signal, and then the subwoofer reproduces the bass present in this signal. This connection also has a crossover and gain control, and in the case of the high level connection, this is important, as there are no controls for this for this connection method in a normal processor. The gain works the same as for the low level connection, and balances the output of the subwoofer to match the rest of the system. The cross over also controls the crossover point at which the subwoofers frequency range crosses into the frequency range of the speakers. The main use for the high level input is for using a subwoofer in a stereo system, or other non bass managed system. I'll try and explain the differences in how these two connection methods should be used. To do that though, we first need to understand how the channels are encoded for a digital soundtrack, so I'll cover that for the benefit of anyone that doesnt fully understand this first. If you know this stuff, you can skip the next paragraph. In a normal full range speaker system the full frequency range is sent to the front left and right speakers, and any other speakers that the system might have. With a digital sound track, this remains unchanged, but we have the addition of the .1, which is the LFE channel. In a multi channel system, the speaker channels still have this full bass content in them, as well as this LFE bass channel, but the processor gets involved. When you tell the processor you have a subwoofer connect, the processor will filter out the bass content from the speaker channels, and re-direct this bass to the subwoofer. This is what the speaker crossover settings do. The result is then that the subwoofer is responsible for playing this re-directed bass from the speakers, as well as the LFE content. So, how to best use the connections. Well in a typical bass managed setup, like a home theater setup, there isnt really any need for the high level connection, as the low level connection takes care of this by means of the redirected bass being sent here, as well as the LFE content in movies. This doesnt mean that the high level cant be used in a HT system, because it can, but for it to work, the system has to be used a certain way. The high level connection requires that the speakers get a full range signal, so to use the high level connection method in a bass managed setup, you must ensure that the speakers receive a full range signal. Typically, this would never happen in a home theater setup, so it requires user input to ensure this does happen if you want to use the high level connection method. This might lead you to ask if there is any point using the high level connection, but for some there might be. Firstly there is a subtle difference between the crossover typically used in the high and low level connections. In simple terms, the crossover for the high level connection will usually use a more gradual slope, which in theory could lead to a smoother integration between sub and speakers, where the speakers are full range. This leads to the second important point. Speakers being run in full range mode will not crossover according to the setting in the processor, instead they will output material down to what ever frequency response they are capable of, so this is why the crossover on the subwoofer for the high level connection is more important. The crossover frequency should be matched to the natural low end frequency response of the speakers being used, and this will vary from speaker to speaker. If you are using small sats, then there seem little point in using the high level connection, but if you have larger more full range speakers, then for music this may be preferable. In my own experience, larger speakers will have their character changed when filtered by a processor at 80hz, and this can impact music negatively. If this is the case, then running the high level connection can restore the speakers natural character, while still allowing a subwoofer to improve the low end response, and the end result can be positive for musical listening experiences. There are two main ways I have seen high level connections offered. I have owned subs with both but only ever used one. The first is via a single input using a Speakon connector. This will read the signal from both the front left and right speakers, and produce the bass content in them upto the crossover frequency you select on the sub. The speakers have no filtering provided for this type of connection so will run full range. The second is with speaker terminals on the subwoofer, with an input and output. This type of connection allows you to filter off the low frequency response of the speakers off if you choose. As the signal from the amp if fed into the subwoofer, the bass is produced by the sub, then the remainder of the signal is fed to the speakers. This essentially provides the same function os the crossover settings in an AV processor, so the low end frequency response of the speakers is filtered off with this setup according to how you set it on the subwoofer. Hopefully that should cover the issue well enough to explain it, and allow people to decide on if the high level connection is for them. Hopefully it will also resolve some of the confusion as well. If there are any glaring errors anyone can see, feel free to add to the thread and I will make corrections where needed.