I'll try the best I can with only a little bit of time with T90. I actually haven't had my RS1es very long either, but got a lot of listening in.
Let's start with Bass. I've read a lot about people write about bass, generally, in a way I think I've misunderstood. Many people complain about the bass response of Grados and I never understood that. I think I thought of bass as sort of an add on to the music. Like, only something you need if you're listening to hip hop or electronic music (which I do rarely). So, I sort of wrote it off as something that didn't interest me much. It's also worth noting - when I talk about bass, I mostly mean sub bass. The type of bass that you can tell is void from Grados response curves. The mid bass is actually pretty decent and punchy on Grados - especially on the PS500. (No experience with the PS1000)
Then enters the T90s. Wow... I was missing a lot in many songs I love and enjoy. There are entire sub bass parts that I just never knew existed. And, the quality of the bass all around on these T90s is just amazing.
So, there's some specifics on the T90 bass vs the general Grado sound. To me, the RS1s are just a sort of high quality archetype of the Grado house sound. I have a pair of MS1i's that I like and they get me about 85% of the way to the RS1. They're great phones, but after I heard the T90s, I could tell they were just on another level. Or, perhaps for another purpose. I know we all shy away from saying one headphone is better than another (they're just different, right?) But, I can't help to have the opinion that the T90s are just an altogether better headphone in many ways. I continue to think this is a great upgrade path for folks who want to experience headphones outside of the Grado line. You can have your nice sparkling, high res highs that Grado listers are used to, and then benefit in every other aspect of the listening experience at the same time.
With all that said - I still think the PS500s are something really special in the Grado line. Everybody needs to give those a listen.
Anyways - maybe that helps shed some light. I know I didn't talk too much about the specifics of the RS1s, but I'm not great at translating sound into words as some of you can.