the RX-40 is a very light-bodied guitar (resoncast). i have to supplement it with the tone zone to make it sound thick. the original default humbucker was treble excessive, it made the guitar sound shrill.
the current GRX-40 has agathis body which is a more resonant unit but the default humbucker has excessive midrange IMO. feel-wise, there's not much diff between the 2. i like the fact that the current GRX-40 sports a more palm-friendly bridge- no protruding screws to jab your picking hand.