I shall review the larger Usher X-708 before the S-520, as it is a no brainer.
My main speakers are the 15" Tannoy Canterbury HE's, which can be driven by SET amps to concert hall level. However I do try out various small and mid-sized speakers for fun. The problem is, after you get used to large woofers and full sound it can be difficult to listen to things that seem unrealistically small, and that's not just a question of frequency extension. The good ones, like Proac Response 2 (original), ATC20, LS5/12, even the smaller ATC7/A7 and Audio Physic Step, can still deliver great presence with less challenging material, but when the full Mahlerian orchestra comes in, you know what's missing.
In this respect, X-708 is a class-leading (meaning size, not price) performer. It sounds less compressed that almost all the aforementioned speakers I have on hand. On Tennstedt's live Mahler 5th with the CSO (EMI Japan) you can hear into the crescendo's and feel the continuous increase in volume (the CD player has to be able to do this too; many don't). Images are surprisingly full; one would think one is listening to much larger speakers. The full orchestra is superbly well laid out. The soundstage is unusually deep and the X-708 also possess the rare virtue of exceptional front-to-back layering.
Piano replay is yet another strength of the X-708. Unlike most monitors of equal or smaller sizes, the piano is realistically large, not a little thing. The full body is conveyed as well as the percussive leading edge. This doesn't mean the violin is slighted; its size is appropriately smaller.
Given the superb classical music playback, what about jazz? Patricia Barber's "Touch of Trash" shows off X-708's amazing speed. The percussion galore has the kind of bang-on-a-can vitality and presence that is usually found in only horns. Meanwhile the vocal is kept neutral and enticing, never harsh. The neutrality of X-708 is uncanny, avoiding the sort of paper-thin sound you get in many, if not most, expensive and worthless German/European "high-end" speakers, while at the same time providing a clean and full bass that goes satisfyingly deep.
These are revealing speakers so a little tube is beneficial. I use a tube preamp anyway, but I partner it with a solid state amp most of the time. It's not choosy, whether I use Bryston or Rotel or Counterpoint the sound changes a little but stays excellent. The advantage of cheap speakers is that you can even use them with very cheap and "volatile" amps like Counterpoint SA-12 without fear! Come on over, OTL (don't have one on hand)! With a tube amp, like the 40-watt Copland 501 (EL34 PP) sound is even better, and not soft. I also tried the ss ARC LS-9 and it too did a good job. So, provided you take some care in voicing, X-708 can be used equally well with both ss and tube gears.
Can you get away with less power? I certainly think so! In my >250 sq ft LR, I can use my 300B amp to good effect. Except for things like Mahler, with jazz and vocals you get very full and satisfying sound. This is an advantage over other smaller speakers like LS3/5A, S-520, ATC7 etc. In this aspect, it reminds me a little of the old Sonus Faber Concertino, another excellent speaker which the X-708 resembles a little in appearance!
I now have more than a hundred hours on mine. Initially they can be slightly metallic. Since I plonked them down on my massive clone-Sound Anchor stands (used for the ATC20), situated 2/3 from the rear wall, I have never felt the need to change to any of my other speakers; they are that good. The rear port never invited attention. Several friends with demanding ears have come and marveled at the value Usher provides at each price point. To my own surprise, this one is staying in my system and I am debating whether to get a second pair (to be placed inverted on top) or to get a pair of Be-718 when I next visit Taipei.
Rating: Magnificent! At any price!
Advantages: Big big sound, Neutrality, Dynamics, Speed, Bass.
Disadvantages: Rear port; No dealer in HK.