I've owned my SR1a for 14 months now and can drive them directly with my Jot R or EVO400 with the interface.
To my ears the SR1a sound is not like speakers, but not like typical headphones (open backed or closed) either. It's it's own species in sound presentation, more akin to the K1000(?) and current MySphere models.
Even though the ribbons are dipoles there is inadequate L channel delayed signal to the R ear and vice-versa to set up a good sound field as can happen with most conventional speakers, especially point source or line source dipoles properly positioned per room requirements. But what is there is very stable as is imaging and, I think, captures what the recording has to offer, both good and bad.
But the SR1a's other sound qualities (transparency, ability to capture recording venue ambience, air around instruments and voices - except at the very lowest frequencies, speed of transient attack impact and decay characteristics, tonal reproduction, ability to maintain control of say a 100 piece orchestra going full tilt and keep different groups of instruments differentiated without coming unglued and throwing in the towel.....just some of the things I like about the SR1a and what I think it excels at.
The loudness level of deep bass notes fades as it approaches the very bottom of the frequency range but it is highly articulated all the way down and I find EQ works very well with the ribbon. I find the SR1a can,with some recordings, generate a bit of air, but often not enough, to make say a bass drum or large tam-tam stroke approach what I feel is comparable to what I experience in the concert hall.
It takes a little time to adjust the mechanical aspects of the SR1a for best fit and sound, especially the wing flare. But once I found my sweet spot for the mechanical variables the design holds those aspects nice and tight so I'm not readjusting for every listening session.
These are extremely clean sound transducers and the tendency might be to play them louder than is safe so that is something to be aware of.
I also find it comfortable to wear, and there is no sweat inducing heat buildup to deal with.
I do look forward to putting an HSA-1b thru it's paces sometime the first quarter of 2021 and see what it brings to the overall SR1a soundscape.