Andrew R Stewart 

New pads might be thicker so that the canti arm is not rotating as far for the pad to hit the rim. So less spring force at point of contact and the changed straddle cable angle. Can the pads be positioned closer to the arm? There's nothing wrong with using the extra spring hole in the post base. Also the spring cab be bach bent to increase it's tension, be carefull here though, a little is good a lot is bad. Straddle cables can be changed or lengthened to reaquire the angle that works best. I assume you cleaned the pivot and lubed them while the cantis were off the bike. How are the cables? Do they slide smoothly and without undo friction? One of the advantages of cantis is there wide range of adjustment, one of the dissadvantages is the same... Andy.