I have the ESL 15's smaller siblings, the ESL 11. I'm pretty sure there are huge similarities in sound, with ESL 15 providing more of everything. At least this is how ESL 11 and ESL 13 compare in my experience.
ESL 11 are extremely sensitive to their physical environment as well as to the voicing and quality of the partnering equipment, cables included. They can sound downright bad when these factors are not properly addressed. I don't know if this is true for the older Logans too, but it's definitely true for the current Masterpiece series. In my room and according to my tastes, they have good synergy to Accuphase gear and this is what I am using at the moment. But it's also a matter of taste, my system is particularly good for classical music for example and I prefer a somewhat warmish sound.