An A3 in a '59N will add mids and give a 'larger' sound. The high-end will still be bright, but it will be more rounded, rather than sharp. A3's have less low-end than A5's, so you shouldn't have an issue with boominess. Should be a nice clean tone.
An A4 in a PGB will add high-end and reduce mids. The low-end will be firmer. A4's have a little more output than A2's, but not by much. PGB's are fairly bright to begin with, and an A4 would be adding more treble (as A2's are a warm magnet). Some guys really kick things up by putting an A8 in their PGB, which adds noticeable output and gives a fuller sound. If you want a bridge PU with some muscle, you might an A8 in there.