But, again the servo sub has a big advantage over others as Jeffca mentioned above explained by Brian. I have a different snippet of Brian talking about the advantage of servo over DSP, which is ultimately why I ended up going with Rythmik subs. Rythmik subs go head to head with the other internet direct subwoofers in terms of output at a given price level, but you get the servo design thrown in on top of it. Here is Brian talking about what I mentioned:
"DSP is good, but is not perfect. There was a company that claimed they can make a NHT speaker generate a perfect impulse response. I visited the company's booth in CES and asked the engineer just one simple question, does that mean the true quality of the speakers is no longer important? He refused to answer my question in front of other customers and wanted me to go away. The truth is like this. First, all speakers have characteristics changing between small signals and large signals. What is a bad speaker? A bad speaker is a speaker exhibit wild difference between small signal characteristic and large signal characteristic. Does the DSP fix the small signal characteristic or the large signal characteristic? DSP is a linear operation, it cannot fix both. But servo can fix both. Secondly, you know the car you drive does age and its feel is difference when it is cold vs when it is hot. Why do you think the subwoofer is any different? When it ages, the frequency response also changes. Does the DSP correct that? No. When the voice coil in the subwoofer is cold vs when it is hot, the frequency response is also different. Some called it thermal compression. I prefer to call it thermal memory effect. What is memory effect? If you press a memory foam, you will notice the impress takes some time to recover. The thermal effect on voice coil works the same way. You will hear the term thermal stress very often to explain how a voice coil can be fried. Now think of how the thermal comes and goes in voice coil. Does DSP correct that? No. Some subwoofers have such as poor memory effect, people think the subwoofer sounds slow with respect to the other channels. That slowness is a profound characteristic. Otherwise, all we have to do is move the subwoofer closer to us to correct it. Lastly, we all heard the term "unit-to-unit" variation. I can even extend that to "batch-to-batch" variation. How many times people find the actual frequency response is different from published spec? In fact, these variations do exist and is the explanation of what had happened (assuming the manufacturer does not make up the spec in the first place) . How does DSP address that? It cannot. Do you really think they will give each batch a different DSP correction? That will only create more service problem. As a result, you will see some frequency response measured by third party that does not look like smooth curve at all. It is a clear indication that imperfect EQ has been applied and as a result wavy frequency response is measured, especially when the reviewer pick a random unit from the market place without manufacturer's acknowledgement. It is imperfect because the correction is only valid to the sample unit in engineer's hand. If they find the production units are different from the sample unit, what do you think they will do? You would have guessed it right. Ship it. The customers will buy that DSP rubber stamp. Servo is different. It is a closed loop feedback system. It takes the unit-to-unit/batch-to-batch variation into account and adjust the needed correction. So now you can guess how can DSP generate a perfect impulse response? It can, but only at one particular level that the engineer samples. That is the reason I was sent away because the R&D already know the limitation. If TV is selling that DSP to me, I will challenge the same thing. DSP only works with the assumption of a model. Who double check if the model is correct? If no one does, it is just a rubber stamp. Not only that, you know that two bad characteristics in the system, instead of just one."
