I have never worried about timbre matching, but rather focused on quality. If you can, test your main system center in the wife’s system and see how it works as an example of non-matched.
As far as matching those old JBL, I think you are going to be SOL. However, I have heard great reviews of the Klipsch RP-450C and 504C centers. The 450 I believe is a better timbre match to your RB-61 ii's I believe since the 504 now uses the new vented tweeter. Many of the people I know that run them say they compare favorably to the much larger RC-64 III. There are multiple 450's on Ebay weekly from some of the top retailers that consistently sell this center for under $400 with shipping. Crutchfield also has them for $399
