OK i have a question, probably the best 3 subs on the market .
Martin Audio Sxh218 
Hennessy sound design battleaxe
Istthere a clear winner here and what are your opinions. 
These subs seem to be super amazing. 
Unfortunately I'm in Australia and don't get the opportunity to listen to these. 
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I can’t speak to the Martin but I have experienced both BassBoss and Hennessy. Hennessy is horn loaded so the boxes are huge and heavy and still don’t go as low as the ZV28s. BassBoss is amazing gear and will blow your mind. Price size output the ZVs take the cake. The battleaxes are wonderful as well don’t get me wrong but I would take ZVs over them 
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David Lee

The BASSBOSS ZV28 is by far the deepest performing subwoofer on this list. It’s also the only direct-radiating vented enclosure on the list. It's also the only one for which a frequency response plot has been published. It's also the lightest subwoofer on this list. It's also the only powered subwoofer on this list, making it the easiest to set up and the one with the comprehensive, built-in limiting. Factoring in the price of amplification and processing, I'm guessing these are probably the least expensive as well. So far so good.  
The Martin Audio SXH218 Subwoofer is a hybrid horn/vented enclosure, more similar to the BASSBOSS VS21 than the ZV28. The details that are immediately noticeable as different are that they don't include power, that the peak SPL is calculated, not measured, and that the sensitivity is based on the standard 2.83V into 4 ohms. What’s wrong with that, you may ask? Well, if the input voltage was actually 1 Watt, aka 2V into 4 ohms, the sensitivity would be 104dB.  
Assuming that the conversion was done on the basis of 1 Watt being the starting point, specifically, 2V into 4 ohms, to reach the “Calculated Peak SPL” would require… wait for it… 25,000 Watts! Even if they could get 107dB from 1 Watt, which they don't even claim to be able to do, the power required to reach 148dB would be 12,500 Watts. So the claim of 148dB peak output is essentially impossible.  
The 107dB is a 2-watt starting point and the amplifier they sell for the box is rated for a maximum of 10,000W per box when using 2 channels bridged per box. Using their own specifications, without the “errors” the box would, at best, and without factoring in any thermal compression, reach peak output at 144dB. Now that's not bad, but apparently not good enough for the marketing department.  
Now we all know that no voice coil, or even pair of voice coils, can handle 200 Volts and 50 Amps for very long because things will get very hot! Heat causes what's called Thermal Compression. That much power will cause quite a lot of Thermal Compression. The real-world result of putting 10,000W into a speaker and getting 144dB out of it will be that the voice coil gets hot in a hurry. The second time, and every subsequent time you put power into the speaker, less sound pressure will come out of it.  
It's not unrealistic to expect about 3dB of loss due to the heat building up in the voice coils, especially when being hammered with such large amounts of power. The question to ask is, given the really optimistic nature of the other specifications, how long would you expect this speaker to last if you asked it to do anything like what they claim it can do? And, given those same dubious specifications of SPL capabilities, what are the chances that their frequency response specifications are similarly unrealistic?  
The Hennessy Battleaxe Subwoofer is also specified to have a 107dB sensitivity and it has a single 4-ohm woofer, meaning it's likely that the same standard was used, i.e. 2.83V into 4 ohms, so effectively 107dB at 2 W at 1 M. Again that's 104dB at 1W at 1M in the real Ohm's Law world.  
This design has the magnet in a sealed enclosure, and all the power needs to be dissipated by one voice coil. The questions I have are, at what frequency is the Battleaxe that sensitive? And how does the design allow the voice coil remain cool enough to survive hours of pounding at a festival? What's the failure rate and what's the warranty?  
Their specifications are masterfully vague, stating “an intended passband of 28hz - 100hz, employing a 24db/octave 25hz buttersworth hipass“ (sic), which says absolutely nothing about the output when said filters are in place. (Just the fact that the notation errors are so bad makes me wonder if they know what they're talking about. The filter topology is called a Butterworth filter, after Stephen Butterworth, and it's capitalized. The abbreviation for hertz is also capitalized, Hz. And the abbreviation for Decibel is denoted as dB, small d for deci, or one-tenth, and big B for bel, as in Alexander Graham. They do manage to capitalize the B in Battleaxe, so we have an idea of where they put themselves in the hierarchy. 😉  
This attention to detail doesn't instill confidence in their specifications any more than the questionable math in the Martin Audio data sheets. At least Martin Audio includes the numbers in their data sheets, even if they expect nobody will catch them in their BS. Hennessy is smart enough to not publish anything that they can get called out on. I've no doubt that the Battleaxe is loud, but they make a point to note “with optimum sensitivity being achieved from a block of 8”. I'd suggest that a block of eight ZV28s would deliver some pretty “optimum sensitivity” too. The difference would be in how the crowd experienced the sound.  
The ZV28s are comfortable and powerful operating below 30Hz. They are about 3dB less sensitive than the other two options BUT their sensitivity is consistent down to 28Hz, They are less violent than horns but incomparably satisfying when depth is called for. The nature of boxes that are not comfortable operating at very low frequencies is that they tend to strain when called upon, or they just don't deliver those notes at all.  
These other cabinets also tend to deliver the higher notes, where they are comfortable, much more efficiently and effectively, as in louder than the low notes. The low notes are audible, but not as powerful, and any notes near the system's comfort-zone get exaggerated. For the amateur festival attendee, it's fun to feel a system hit you in the chest. For those who don't want to be flogged with the SAME notes hour after hour, night- after day- after night, the ZV28 introduces an aura of wonder and awe as notes that most people have never heard before wash over them and energize different parts of their bodies and minds…  
Yes, asking the designer of one of the loudspeakers which box is best, you'd expect to hear a biased answer, and you'd be right, but in this case I'm biased against the bullshit. Practically speaking, you can't bring the boxes together and have a listen, and I've not been able to do so either, but, based on the quality of the information available, it's pretty obvious which option is likely to deliver what it promises. BASSBOSS delivers bass, not BS. If that's something you can appreciate, you'll definitely appreciate the ZV28s.
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