Your LBS might have been thinking in the other direction. A 559 mm (MTB 26") tube can be made to fit a 26 x 1 3/8" rim, with some stretching (I know because I made this work for a couple of months), but I just don't know about putting outsized tubes on smaller rims. It might work, but then again, it might not inflate evenly or well, which could be problematic for a number of reasons. Why on earth did they give you a 590 mm tube anyway?