Upgrade ARC preamp s to LS-26 ??

I have owned an SP-3A-1 since 1974, and in 2003 I had it upgraded to an SP-3c. I also own an LS-15. Lately I've gotten the bug to upgrade. I am thinking of selling both, adding a few $, and buying an LS-26 (used). Any thoughts ? Would the LS-26 be a big upgrade from either the SP-3c or the LS-15 ?
I owned an LS3B for years. I occasionally brought home other AR preamps to demo, including an SP16 and an LS25II. Never felt moved to replace the LS3B. Then, I demoed the LS17 and heard coherency, delicacy, dimensionality and fidelity I hadn't heard in previous models. Couldn't afford a Ref3 but auditioned the LS26 and was smitten. I bought one and I've never been happier with my electronics. Can't speak specifically to the SP3A or Ls15, but the LS26 is a gem.
77jovian: I'm torn between the LS17 and LS26. Can you comment further on what you heard comparing the two? Also, can you tell us a little about the system you heard them in so I can get a better frame of reference?
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.

My system:
AR VT100iii
Ayre C5xe
Thiel 3.6
Cables are Cardas Golden Reference and Tara Labs, and Jorma Design power cords.

Good luck with your decision.