I was tempted to save the $$ and buy the LS17 instead of the LS26. The LS26 is worth the difference. I'm not good at the lingo, but I'll try. Here's what the LS26 brought me. First, I noticed instruments with extended range (piano, cello, for example) sound like the same instrument from bottom to top of their range; not chesty in one octave and brittle in the next. Next, I noticed percussive effects; triangles, cymbals, plucked strings, etc. hanging in three dimensions, treble is extended but not tizzy...gorgeous. I've never been so conscious of the richness of microdynamics, detail and resolution. The soundstage is wide, deep, and stable; I feel like I could walk around between the instruments.
There's a good discussion about the LS17 and LS26 on Audiogon, the link is below. Other people are better at putting their observations into words. Here's my best effort. The LS17 is 12-year old Macallan; a very satisfying single malt. The LS26 is 18-year old Macallan. Same family, but the 18-year old is richer, more complex and rewarding in every way.
The features of the LS26 I have come to appreciate most are fully balanced inputs, switchable 3-stage gain, polarity, large display with adjustable brightness, and all the features are available on the remote.
Cables are Cardas Golden Reference and Tara Labs, and Jorma Design power cords.
Good luck with your decision.