Even though it will cost us the most money, honestly I think upgrading the 5Bs to the 5Ts will give you the most marked improvement as the extra 5.25" woofer does make a big difference in the speaker's output and soundstage, which I believe are your current concerns.
However I do think that the 805 is better suited to Aperions than the Yammy, so I would still keep this upgrade as a future consideration and I bet you can get most of your money back for the 661 on the secondary market as it is a popular model.
Jason Hicks
Aperion Audio Guru