Another ARC LS-26 owner here, othewrwise know as a "Baby ref 3". Given both the 17 and the 26's low output impedence there should little to no matching problems with your amplifier. The sonic signature is ever so close between the two ( both employ a pair of 6H30 tubes in the gain stage along with JFET/hybrid audio circuit). Tube life "should" be long, 5000 + hours (FWIW, I did have one of mine go microphonic @ 1000 hrs)
Outside of better display on the 26 , IMO, the sonic improvement is summed up like what what stated before me.....another vail being somewhat lifted. I came from the 16 and when auditioning the 26 side-by-side with it there appears to be better instrument localazation as well.
In summmation, both would perform well, hopefully you can let YOUR ears decide !