I don't mean anything technical by timbre! By timbre I just mean generally how it sounds like regardless of pitch—for example, a violin and a viola are similar musical instruments, but have a different timbre, in the sense that the viola has a deeper, richer sound even when played at the same pitch as the violin. And a tenor singing sounds different from a bass singing even when they are singing at the same pitch. That's all I meant by it. When you are trying to use "different voices", you are changing the timbre too.
It's my opinion that you don't really need to worry about the English R
I often chat with lots of Europeans / South Americans speaking English over the Internet, and almost none of them use the English R and just use the R from their own language, and it's perfectly understandable
The English R is very rare, so it can be hard!
I also think that one should reduce the resonance in the chest instead of getting it out "completely". Leaving some in the chest makes it sound more natural than having it completely out. My chest is always vibrating to some extent when I speak.
PS I tried to do a recording to try to show different-sounding voices at the same pitch, but I failed miserably at it... have a laugh though if you like! Like I said I use quite a low speaking voice...
http://www.sendspace.com/file/t88f9l (zip password aepfelsindyummy)