Ok, I am back from the Stax dealer. I had my listening session with the L700 for two hours in very good quiet conditions (a speakers listening room with the door close).
The source was a Moonaudio CD player NEO 260D with DAC. Cables were apparently high end too...
I tried the L700 and the 207 on the SRM-252s and then changed for the SRM-006ts (that was the only other Stax amp available).
First conclusion : the L700 are much more confortable than the SR-207. Pads are buttery soft and the new adjustment system is more practical to keep the good adjustment. With the synthetic 207's pads I start to sweat after a certain uninterrupted time of listening. That didn't happen with the L700. Weight feeling is about the same (I din't check the weight of each unit...). No farts on the L700. I wear glasses and the seal was better with the L700 (remember, buttery soft pads...)
Second conclusion : the SRM-252s is more than capable of driving both headphones! Volume was at 10/11 am for both headphones and the music was loud enough for me. At 12, it started to be too loud for my pleasure, with the SR-207 or the SR-L700.
Third conclusion : I didn't hear (emphasis on "I") any differences between the SRM-252s and the SRM-006ts, both with the 207 or the L700.
I don't pretend that there are no differences, I am just saying that I didn't hear any!
Fourth conclusion : the L700 are (for me) better than the SR-207. Everything was better in my opinion... Bass, sub-bass, imaging, soundstage, etc... How much better? I would say by 15/20% (if that makes any sense to estimate the differences by a %). Which mean, nothing to
rationnaly (BUT, is there anything rationale in that hobby??) justify 3x the price difference!
I think that the SR-207 are a real bargain at their price! With the SRM-252s you have a killer system. I also think that the L700 are also a bargain BUT you have to compare them to the "high end" models (SR-007/SR-009). In that league, they have the best Q/P ratio.
I had the SR-007MK1 with the SRM-717 for more than 10 years (my only system for a long time) so I think I can give my appreciation (not to compare, since I didn't had the two headphones at the same time) between the L700 and the SR-007. I think that the differences between the SR-007 and L700 are lower that 10%.
Now, I am debating if a 15/20% differences can justify the price difference for me...