Owned the MS1, SR325i and RS1 at different stages. Have heard most of the SR Grado line-up.
Grados IMO = intimate sound stage (until you get further up the line) - often like being front row or on stage with the musicians, very mid-centric, especially upper mids, bright presentation, most of the bass is mid-bass rather than sub-bass, very fast.
DT880/T1 = wider and more compelte soundstage with great imaging, a lot flatter frequency response (rather than the elevated mid-range of the Grado), more cohesion in the bass (sub to mid), airier presentation that is quite bright. Unbelievably more comfortable - but often need additional amping - where the Grados typically don't.
Hope that helps.