I think the SR-007 has pretty good bass, but electrostatic drivers can't be allowed the excursion possible in a dynamic or orthodynamic and so even the wonderful SR-007 just doesn't have the "punch" possible with dynamic/ortho drivers.
My SR-007 and (to a lesser degree) my Lambda Pro Signature have deep, tuneful, textured non-boomy bass, but they lack that nth degree of 'slam.' The Koss ESP-950, with very large diaphragms, do manage to come a bit closer to HD-800 / LCD-2 'slam' but don't quite get there either.
The Lambda Pro Signatures are so "fast" sounding, though, that upper overtones of bass activity take on a special quality that my hearing seems to "backfill" back down into the bass, and so while there's not as much actual "impact" there is a kind of apparent bass detail and "speed" that gives a very nice quality to their bass. The ESP-950's aren't as detailed as that, and although their bass is tight sounding, it doesn't have as much 'speed.'
There is a kind of wonderful sonority to the mids on the Koss ESP-950's, or midrange detail, or midrange texture- I can't quite put my finger on the exact quality, but there something I like about the way the mids sound on the ESP-950s, that, along with reasonably extended and well-behaved highs and the decently low and tight bass, make them sound pleasantly different from the Stax. They present the mids differently than the Stax, and compliment these well-articulated mids with bass and highs that don't misbehave. And, on some music, I do get a good dose of that "hear through" quality on the Koss that the Stax produce in abundance.
I wonder what it would sound like if Koss did more R&D and improved the ESP-950's to bring them up a few notches. That might be quite a headphone. However, an even higher-end electrostatic headphone is not a product that Koss would likely reap big profits from, due to the fairly small market. Koss is a pretty large outfit, with lots of overhead and marketing costs, so I imagine they're spending their R&D to come up with products that will sell zillions of units in the mass market, not tooling up to make uber-electrostats for a handful of obsessed headphonophiles.
By the way, I did put in a summer of work at Koss in 1976, between college semesters. I did not work with their headphone stuff at all, I did a variety of odd jobs for Koss such as wiring up John Koss's new house with audio gear and TV and FM antenna signal distribution, and also worked with their Acoustech division, which was all but dead by that time. The last remaining engineer at Acoustech had me fixing some of the last Acoustech power amps that were still in use, which would come in for repair from time to time. And I remember having to sort through a big pile of output transistors, making sure that none of the stock we had of these old devices were open or shorted. Also, he had an idea to design and manufacture a "student" model of an electron microscope. I wired up some prototype circuits that he sketched out, and we were trying to use some high voltage low-current transformers they had left over from Acoustech electrostatic speakers to breadboard early concepts of this electron microscope idea. Mostly what we got was smoke and some sparks, the circuits were very early in the design process of the thing. I left and went back to university, and as far as I know nothing ever came of the "budget" electron microscope. This was at the same time that Koss was selling Model 1, 2 and 3 Electrostatic Speakers. I kind of liked the Model 1, but the others did nothing for me. Maybe there was also a Model 4, I don't recall. They tended to run the Model 1's with G.A.S. Ampzilla's and they sounded OK I thought. More bass, and generally louder than the KLH 9 which was the only other full range electrostatic speaker I'd heard up till then. I knew about Quads and Infinity Servo Statik's but I never heard any of those.
These were certainly not the "good old days" - I am much happier with the audio gear I have now! But, I DO miss having hair.....
