I think not that the PR has no value, no way and i didn't know whether the blogs(Subdomain) are aged or not but i think it doesnt matter at Tumblr/Web20 Accounts. Thank you.
This is really a Great Case Study.. will definitely follow you.. But I have a Question.. As you said your Using PR2 Tumblr and now as PR Has no Such Value so it should be Done without PR Blog but Aged Blog.. Isn't it..?