I'm going to preface my response by saying that I'm just one guy and my experience is unique to me...m'kay?
I, personally, had a bad experience with Epik and while the subwoofers that I bought from them (Sentinel, which is no longer offered) didn't quite live up to my own, very picky, standards, they did a couple of things extremely well.
I have also owned an HSU VTF-3 MK2 (very similar to the VTF-2 MK3, just with 100Watts more power) for many years now. I had a much better buying experience with HSU and I also preferred the VTF-2 MK3 over the Sentinel, even though I was purchasing the Sentinel subs as an "upgrade".
As a short summary, the Sentinel subs did play lower - which was the only thing that I felt was lacking with my VTF-3 MK2. But the Sentinel had noticeable over-hang - notes did not stop as quickly and the driver continued to move after the signal had told it to stop. Basically, the Sentinel could "rumble" more and lower than the VTF-3 MK2, but it had less control and didn't sound as "tight".
I also had issues with the quality of the components used in the Epik sub. The amplifier seemed like a really cheap, bulk-made unit. The power cord fit loosely, it produced a very quiet, but still audible power transformer hum, it got hot, and Epik seemed to have dampened much of the somewhat higher bass frequency output. In other words, I had to turn the volume dial quite high in order to get the same 45-80Hz output as the (spec wise) less powerful VTF-3 MK2. Where I had the VTF-3 MK2 dial turned to about 11 o'clock, the Sentinel had its dial turned to about 2 o'clock. Given that the Sentienl could play lower though, I believe Epik simply "limited" the amp at the higher 45-80Hz frequencies so that it could maintain a flatter frequency response.
I also feel that there are a lot of "red flags" with Epik as a company. First and foremost, they will not ship or warrant their subwoofers anywhere outside of the USA. Look around at all of the other internet direct companies out there and you will see that this is a very rare occurance. I do not think it reflects well on a company that they refuse to warrant any product that leaves the USA. They sell direct, after-all, they ought to be able to stand behind their products no matter where in the world they are being used!
Second, Epik continually discontinues and introduces "all new" models. If you look at their current lineup, they only offer two models and NONE of their older models are available any longer. It isn't so much that I have a problem with a company introducing new products on a fairly quick and regular basis, it's the discontinuation of all previous products that strikes me as being a bit strange.
Here's the thing, you can't find professional reviews for any Epik subwoofers. And as soon as any reviews DO come out, Epik discontinues that model and introduces another one. I was not the only person who had issues with the Sentinel. Several people complained about the same amplifier problems that I had. As soon as word spread about those issues, the Sentinel was discontinued. It just strikes me as shady is all.
Third, Epik rarely has any product actually in stock and ready to ship. Their products are almost always in back-order. And when they are shown as being available for sale on the website, they aren't actually "in stock". Instead, when you place your order, your subwoofer is put in and order line and physically assembled some 3-7 days later. Products show up as actively for sale when Epik has the PARTS, not the completed subs ready to ship.
Finally, there was the way I was treated. On average, I had to wait 3 days for any reply. When I did get a reply, I was told to be patient and wait longer for further information. When I had a problem with the amp, I was told that there were ZERO replacement parts available, that a new shipment of parts from China was supposed to arrive in two weeks and that there was nothing that could be done until then. I was promised a reply when the parts came in. That never happened. It took many attempts and a lot of waiting to ever get any response and I eventually gave up because I was never able to get a resolution to my problem.
Like I said though...I am just one guy and that was what happened to ME. There are many, many happy Epik customers and the subwoofer itself was not bad, it just wasn't quite what I was hoping for or expecting. I am not familiar with the two existing Epik models, but given the experience I had, I'm not planning, nor eager to ever try buying from them again.
Bottom line, Epik is a very small company that is competing very much on price. That leaves them extremely small margin and they seem understaffed and constantly "behind". I have similar criticisms of SVSound - although not as dire. I find it odd - verging on unacceptable that SVSound virtually never has any of its products in stock. Everything is always on back-order and they NEVER hit their target release dates. It's a "hurry up and wait" sales pitch and I hate that kind of approach. That said, I think very highly - much more highly than Epik - of SVSound's final product quality and performance. No dodgy amps and really, really great performance. And they response very quickly to any communications...they just don't keep product in stock!
HSU, as a company, has been an absolute pleasure to deal with - for me anyway. Their products are RARELY ever out of stock and communication is fast, helpful and very pleasant. They ship and warrant their products anywhere and they will figure out your final landed cost - duties, taxes, brokerage, everything - and offer a very accurate quote.
The only criticism I have of HSU is that Dr. Hsu designs his subs to be very efficient, very affordable, very reliable, but they do not truly troll the depths of 20Hz bass, or if they do, they can't do so with very loud output. The ULS-15 is easily the best HSU sub, but it can't play very loud. That is why they offer them in packages of 2 or 4 with discount package pricing - which is a reasonable way to increase output when extension and linearity are already very good, but output is the only thing that is lacking.
It's a really tough market and the competition is crazy fierce. Another geat ID subwoofer company is Rythmik, but they face many of the same challenges. Many of their products wind up being on back-order.
I gave this reply because you asked for thoughts and opinions on Epik and HSU. I think that both offer quality products. Both certainly offer extremely competitive prices. But I think more highly of HSU as a company simply because of my own personal experience. I don't like the way Epik does business - there are too many "red flags" and it just makes them seem a bit dodgy and shady. SVSound and Rythmik are better (at least they ship and warrant outside of the USA and they respond much more quickly and more helpfully to communications), but they still have issues and I hate they way they have so many products on back-order and try to rope in pre-orders before they can physically deliver.
HSU, in terms of service, communication and having products in stock and ready to ship, I've found to be great! I just wish their subs would play lower and with more authority.
I hope this has been at least a little helpful
The bottom line is that none of these ID companies are perfect. You pick the shortcoming that bothers you the least in order to enjoy a higher level of performance for the price paid vs. what you could buy in a store from a retail company like Velodyne, Paradigm or JL Audio.
Myself, I was EXTREMELY impressed with the new SVSound PB13-Ultra DSP that I heard. It is a level of performance that is unmatched by virtually any other subwoofer I have heard and for a price that is at least somewhat "sane" and within reach or a "normal" person. For that kind of performance at that price, I would likely be willing to wait a few months for a new container of them to arrive from China. But if I needed a sub RIGHT NOW - I'd buy it from HSU. I'd buy the ULS-15 and I'd just buy 2 or 4 of them if I needed the extra output
