well the 425 woul db an improvement os your E4 is more equvilent to the 315 being singeled drivered.
since you dont mention isolation then you may care to look at the Gr07 as its a tremndous all rounder but its a dynamic so the isolation given is far less. things like the R-50, q-JAYS, and GR01 are also options, but again are more of a sideway move than being actually "better" the GR01 may have a the bit more low end your after, not as much as the 7 of course but with greater isolation.
if i was you though, unless you want a quite significantly different sound signature then the sift up to the 425 is probably going to do you well. still you may want to have a read of my reviews on the R-50, GR07 and GR01 to better gauge where i think they all lie in comparison.
or you could go with a PL-50 to get what you have now and spend the other US$100 on something V totally different.