I do own a set of these, and I also have a home theater setup with a relatively large subwoofer. What I can say is that for the vast majority of music, the LX521 in a correctly sized room with placement as prescribed do not need a subwoofer. You are able to see significant movement in the woofers if low frequency signal is present, so it’s possible to see if they are trying to produce low bass and are failing. One thing they do not do as well as a large sealed box is giving the tactile sort of pressurization from the loud bass. They can play loudly, and they can product a lot of nice bass, but there’s something a little different about how it feels on your body. I have not been able to figure this out with in-room measurements, but remain eager to learn more about that.
They do need at a bare minimum three feet of space behind them, but work better with five. I have been able to shoehorn them into an 11x16 foot room with acceptable quality, but they do significant benefit from being set up in a room at least as large as described by the designer, I believe 180 square feet is the minimum.
I am not sure what to say about the ground loop problem. I use the RCA outputs of the 4x10HD and have no issues with hum or buzz.
Thanks for a response from someone who actually owns these speakers. The comments I've seen from different users over the years, with both this speaker and its predecessor, is that the deep bass is probably not adequate to satisfy people who want home theater level of bass, but adequate for people who listen to music. I would be in the latter category, however I do sometimes listen to music where the deep bass is an important part of it and it wouldn't sound very satisfying if the deep bass wasn't fully present and accounted for.
For anyone who isn't thoroughly familiar with the history, the predecessor of this speaker, the Orion, first came into prominence thanks in large part to the praise heaped on it by Peter Aczel, the publisher/editor of The Audio Critic. Criticisms of any speaker design are often based in the understanding of the theoretical distinctions between the given design and more conventional designs. As such, the criticisms of the Orion and the LX521 are generally twofold, and are essentially the same as with any other dipole radiator (or any speaker that approximates a dipole radiator). Some people question whether it is desirable to have so much acoustic energy reflecting from the wall in back of the speaker. Linkwitz obviously believed that this is to be desired, and while he did also mention that the radiation pattern avoided much of the side wall reflections, this benefit seemed sort of secondary, at least less important than the strong desirability of the late reflections from the rear wall, and he emphasized the importance of tonality match of those reflections to the direct wave.
The other criticism that is based in the theoretical understanding of the design approach is with the fact that as frequency moves lower and wavelengths get longer, the cancellation between the front and rear wavefronts increases. I recall that he wrote that the cancellation yields a bass rolloff that increases at -6dB per octave. But I suspect this is fully correct only for a middle region where the wavelength is not a whole lot shorter than the baffle width and not a whole lot wider than the baffle. At frequency high enough to where the tweeters are highly directional, there will not be appreciable cancellation between the front-facing tweeter and the one facing the rear. For upper treble the dipole pattern is achieved merely by virtue of a pair of highly directional tweeters aimed in opposite directions.
At the other end of the spectrum, I think there must be a point where the wavelength is so great that cancellation between the front and the rear is essentially perfect, such that the effect would not become appreciably worse as the wavelength increases further. The only way I would expect to hear much deep bass from any dipole radiator would be if I were sitting close enough to the speaker that the wavefront coming directly at me, from the two woofers, will be greater in strength than the wavefront that reaches me by propagating around the baffle from the other side. The wavefront that comes from the far side of the baffle has a slightly longer distance to travel, and spreads out more, and is thus slightly weaker than the wavefront that travels a shorter distance to reach me. But when you listen from a distance where the difference between the two distances is minor in relation to the distance, this effect goes away. For any listener a reasonable distance from the speaker and for wavelengths at least a few times greater than the baffle width, it seems likely to me that the wave cancellation would be essentially complete. I would not expect to hear much of anything below 100 Hz unless I were sitting within a few feet of the speaker and directly in front of it, not off to either side. It need not be said that Linkwitz understood this perfectly well. This was the reason that he designed and built the subwoofer, the Thor (if my memory is correct he did the Thor when he was still focused on the Orion, prior to the LX521).
The reason I have just now written all of this is not to disparage the speaker. I was curious about whether people who own the speaker are satisfied with the deep bass, and the answer from one person suggested an absence of understanding of the unique characteristics of the design. So I thought I would take the time to write a few words about it. For what it's worth I'm a great admirer of Linkwitz partly because of his independent spirit. But I suspect that if I owned either the Orion or the LX521, I would also be an owner of either Thor or some other subwoofer that I thought would do justice to the Orion or LX521. In the Orion/Thor crossover he designed, the handoff point was surprisingly low, around 50 Hz or 60 Hz. I would be inclined to do this differently, choosing a subwoofer known to give a flat response to somewhere up around 200 Hz. For this particular application I would be inclined to use shallow, 1st-order slopes, so that the Orion or LX521 gradually yields to the subwoofer starting where the wavelength is already several times greater than the baffle width, because at this wavelength (about 5.5 feet at 200 Hz), the excursion of the drivers will already be much greater than it will be for the same woofers in a conventional enclosure (greater excursion is needed to make up for the front/rear cancellation and is accomplished by way of equalization).
In writing this it was certainly not my intent to step on anyone's toes and I hope that I've not done that. I just got the sense that some of this is not very well understood and I thought that it might be appropriate to sort of rehash some of it. With every unconventional approach to speaker design, there are fundamental questions that cannot be overlooked by anyone who is naturally curious about loudspeakers and acoustics. With the Orion and the LX521, one of these fundamental questions is concerned with dipole speakers generally, and the question is whether, given that the dipole pattern is inherently unachievable in bass frequencies where the wavelength is several times greater than the width of the baffle, it is nevertheless desirable for the radiation pattern in the midrange and treble to be dipole. I don't have an answer to this question. I only know that it is a meaningful question that wouldn't be overlooked by anyone who is fully objective, and that conventional wisdom strongly suggests that any attempt to achieve the dipole pattern in bass, especially deep bass, would very likely yield a compromised result that is neither dipole in radiation pattern nor satisfactory in the more obvious respect.