I saw in a thread that the guy said someone on his team is very experienced USB but that it’s just more difficult to work with. He said if it is really something people want and if he can expand his team to focus on developing usb technologies than he would include it. Basically said he would have to find a way to size up the company while still retaining his level of customer service and quality. That was what I remember reading. So There’s a possibility that he may in the future. The only issue that I have with spdif is that I already have something that uses spdif so I’m having to swap between them until I get a splitter or switch box. Otherwise in my opinion is that they work just as well as one another. Other than the fact that usb has so higher bit and dsd. Both of course I can live without 
I will share some thoughts… I have L700 and 353X with Chord Hugo 2 DAC/AMP for 2 years and at first I wasn’t impressed by the sound. It didn’t triggered the feeling that I hear more stuff in my music than before. Actually it sounded very muddy, that’s the first thing I hear when I put them on even today.
What happens if you put a low shelf of 5db at 1khz on your headphones. That boosts everything below 1khz by 5db. That’s how SR-L700 sounds at stock. So everything I listen, it has a thickness, but being electrostatic, you don’t feel this “thickness”.
Now the mids, the vocals are shouty, '50 telephone like sound, just like any STAX.
Lower treble… a thing missing in these headphones.
Sibilance ? Yes, it has it and it’s ALOT.
Highs above 10khz ? Very boosted. I don’t notice right away, but it’s tiresome after 15 minutes or so and I didn’t knew why, now I know.
So everything is boosted. Even the bass sound, VERY boosted but being electrostatic, you don’t feel sound pressure. None. Even in sibilant songs, you don’t get feeling of hearing sound. It’s like hearing sound without ears. On first seconds, you notice right away that the sound is like coming from your room, but soon that impression disappears and you hear it “almost” like a normal headphone.
Also want to try to explain the bass timbre of this headphone. It’s not like a dynamic headphone. It’s like something you never heard before. It’s like a new type of sound. So you are not familiar with this type of sound. And this sound will be in 90% of the songs you listen to. And it sounds weird because you haven’t heard this sound, but it’s 90% of the music you listen to. And everything from 20hz to 1khz, has this type of sound. And being 5db boosted, it sounds OVERWHELMING on 90% of the songs. And this type of sound is everywhere below 1khz so piano strikes, male vocals will be heard merged with this texture, just weird.
Lack of lower treble and shouty mids and “shouty” lower mids/high bass/mid bass and shouty sibilance and shouty treble, make this headphone not cohesive, muddy sounding, grainy sounding, not tight sounding and with a black background missing.
It sounds busy on everything i listen. It has something to do with microdynamics/macrodynamics, a concept i don’t understand. Max settings explains here some of this https://youtu.be/O6nQ1DNhb-4?t=686 It just sound busy and it’s so hard to compare to anything else. For HD800 when you put on, 3 things come to mind: clean, crisp and lackluster bass. When I put L700 on, the 3 things are: boomy, busy, boring. And that with L700 much more resolving than HD800. So I don’t understand how it’s possible to beat a clean/crisp sound with a muddy sound. There are clearly more cues and sounds not heard with HD800, but L700 being less dynamic, sometimes gives the impression of compressed sound where quiet sounds sound much louder than they should. And also this may be a good/bad thing (good for me) that, like I said, everything is boosted, even quiet sounds are much louder, maybe just because of higher resolution.
L700 sounds polite and at the same time overwhelming.
STAX is not the end of the road, it’s just different and only 10% in a good way. 90% is just wrong.
This is the EQ I use and IT’S NEEDED to be able to use these headphones.
Preamp: -8 dB
Filter 1: ON PK Fc 27 Hz Gain 6.7 dB Q 1.05
Filter 2: ON PK Fc 1170 Hz Gain -6.0 dB Q 1.56
Filter 3: ON PK Fc 4024 Hz Gain 3.3 dB Q 0.34
Filter 4: ON PK Fc 5687 Hz Gain -6.6 dB Q 2.98
Filter 5: ON PK Fc 20315 Hz Gain -11.5 dB Q 0.27
Filter 6: ON PK Fc 15 Hz Gain 1.1 dB Q 2.36
Filter 7: ON PK Fc 9141 Hz Gain -0.4 dB Q 2.99
Filter 8: ON PK Fc 11578 Hz Gain 1.9 dB Q 2.62
Filter 9: ON PK Fc 12710 Hz Gain -3.2 dB Q 5.26
Filter 10: ON PK Fc 17988 Hz Gain 1.3 dB Q 3.79
Filters 1 to 5 are recommended.
Filter 6 to 10 are optional but recommended.
Just wondering what dac are you using? And have you considered trying a higher tier energizer?
I have Chord Hugo 2 and Chord Mojo. I haven’t considered a different amp. I don’t think it can change the sound. With eq I like what i hear above 1khz, but the lower mids/bass timbre pretty sure can’t be fixed. It’s sounds different than dynamic and I’m not a fan of it.
Pretty solid 
From my experience it can greatly impact the performance of the l700 and other higher tier stax
Yeah you really can’t do anything to really get rid of the unnatural timbre of the l700, so really any amp nor eq will eliminate it, but a higher tier energizer could help
Wow you make the L700 sound horrible lol. but i dont care for carefully controlled bass/mids/treble. Its doesnt need to sound natural or “right”. All i care about is does it move me? I love aggressive detailed headphones with close sound stage. bloat doesn’t bother me as long as the technicalities are there and it excites me. I would love to hear the L700. You want to sell it anytime soon?