It seems like its been forever since I last posted here! Wish I had more time... To briefly get back to ain holchin b'mamon achar harov, we learned a machlokes between Tosfos and Rashbam whether no rov can be used to determine dinei mamonos or whether a ruba d'isa kaman is acceptable. The machlokes seems to depend on the lomdus behind what ain holchin achar harov teaches: does it mean that rov is categorically an unacceptable form of evidence in the realm of mamonos, or does it mean that rov is unacceptable because it is an inferior form of proof relative to the chezkas mamon of whoever has the money? In other words: is rov no proof, or is rov proof, but not proof enough? I refer you back to this post where we discussed the different approaches of the Ktzos and Terumas haDeshen to this question.
If rov is categorically an invalid form of proof, then the type of rov we are dealing with is irrelavant. However, if rov is proof but not proof enough, then perhaps if we had a super-rov, a rov that was as strong as chezkas mamon, then it would make a difference. It seems that this is the Rashbam's position -- a ruba d'isa kaman is such a super-rov that it can help where rov ordinarily fails.
But is that conclusion true or reasonable? Which rov strikes you as more definitive: 1) two of the three pieces of meat before you are kosher, therefore any piece you take is likely to be kosher (ruba d'isa kaman); 2) the child of a couple living together as husband and wife is most likely the offspring of the husband because he is the most likely to have had relations with his wife (ruba d'leisa kaman)? In the former case we know with certainty that there is at least one piece of non-kosher meat in the mix and are just playing the odds that any piece chosen is not that piece; in the latter case, there is no reason to assume the wife had relations with anyone other than her husband -- in fact, the rov tells us that this probably never happened! It seems, as R' Akiva Eiger writes (Kesubos 13) that the exact opposite of the Rashbam's conclusion should be true -- a ruba d'leisa kaman should be more definitive than a ruba d'isa kaman.
Sorry I am so pressed for time, but I will at least leave a mareh makom to R' Shimon Shkop in Sha'arei Yosher 3:1 and 3:3 who addresses himself to this problem.