The standard answer here is that you want your two subwoofers to be as close as possible in performance. They are, afterall, producing the exact same sounds at the exact same time, so it's theoretically best if they produce those sounds exactly the same!
However, I'm willing to stick my neck out there a little bit and say that it isn't critical for your two subwoofers to be identical. The reason for this is because we humans suck pretty bad at hearing bass! We have a hard time telling where it's coming from, we have a hard time telling what is distortion and what is the original signal, our ability to hear bass at different sound pressure levels varies wildly (at 85dB, we hear bass down to 20Hz just about as well as all the other audible frequencies, but at 75db, where we can very easily hear mid-range and most treble frequencies, we hear the bass as
seeming much quieter than those other, higher frequencies) and we also depend on more than just our actual hearing in order to detect bass - where a tactile sensation will lead us to perceive much "louder" bass, even if the actual SPL isn't any higher!
Basically, in terms of what constitutes "bass quality" the metrics that really matter are: transient response (how quickly the bass notes start to play and how quickly they stop), sheer loudness and extension.
So "matching" your two subwoofers is still a real thing. If you took a cheap, little sub from a HTiB and tried to match it with your HSU VTF-3 MK2, it would be very easy to spot the differences. That little HTiB sub simply won't play as deep, it won't play as loud and it won't have the same tight transient response - so it would make a very poor match and end up distorting your bass when it tries to play the exact same notes at the exact same time as your HSU.
But it isn't vital to get the EXACT same VTF-3 MK2 subwoofer. You just need a subwoofer that has transient response, extension and output that is "quite similar" to your VTF-3 MK2. Your human hearing isn't going to be nearly as picky about the match as it would be for your front three speakers - which are producing mid-range and treble frequencies that our human hearing is quite good at distinguishing
The sub that easily makes the most sense is the HSU VTF-2 MK3. It's the same sub as the older VTF-3 MK2, but just with a less powerful 250Watt amplifier rather than the 350Watt amp in your VTF-3 MK2. Obviously, with that reduction in power, you get a reduction in the sheer output (loudness) that the VTF-2 MK3 can produce, but due to the fact that it takes a doubling of power to produce just 3dB more output, the actual difference in output is really rather small.
On the other hand, if you are getting a second subwoofer purely in order to get louder output (and not so that you can place your two subwoofers in different locations so that you can get smoother bass response at multiple seating locations), then you're going to want to get a second subwoofer that focuses more on output capabilities.
If what you want to do is just make the bass that you already have louder, then you can either replace your VTF-3 MK2 with a single subwoofer that can simply play louder, or you can co-locate (ie. stack or place two subwoofers side-by-side) your exisiting VTF-3 MK2 with a second sub. On the other hand, if what you want to do is create smoother bass response at multiple seating locations, then you are going to place two subwoofers in different locations so that you can create a "virtual sub" whose "image" is located somewhere out in the room where it wouldn't be practical to physically place an actual subwoofer. You can think of it much like how two L/R stereo speakers are able to create a center "image" that makes it seem as though sound is coming from directly in the middle.
When you place two subwoofers in different locations, you don't actually get bass that is much louder. The two subs simply interact with each other and the room and you can use the placement of that "virtual sub" that their imaging creates in order to glean smoother, flatter bass response. If you co-locate your subs, you don't change the flatness of the response at all, you just get louder output - about 6dB louder output if the two subs are identical.
Either way, adding the HSU VTF-2 MK3 makes sense. If you co-locate it with your VTF-3 MK2, you won't get quite the full 6dB boost since the VTF-2 MK3 can't play quite as loud due to its lower powered amp, but it'll still be pretty close. If you use your VTF-3 MK2 and the VTF-2 MK3 in different locations, the difference in output capabilities will be virtually unnoticeable. Meanwhile, the transient response and extension remain very much the same - which is a good thing
Also, the VTF-3 MK2 and VTF-2 MK3 LOOK identical, so from a cosmetic standpoint, it's an easy choice
Hope that helps!
Oh, and don't forget to decouple your subwoofer(s) from the floor! The VTF-3 MK2 fits PERFECTLY on the standard size Auralex GRAMMA. So does the VTF-2 MK3 (since it's got an identical cabinet
).
