I'm not sure what passive bi-amping is, but I bi-amp my Dunlavy IVa with two Plinius SA-102 amps, one for the top posts, and one on the bottom. I have never heard Flmlamb's comment. I have talked to John Dunlavy many times over the years, he helped set me in the direction I went, so maybe I'm just not understanding your question. For my system the two amps are a significant improvement in bass definition and slam. With the fives it may be even more important. I would not go back after bi-amping with the last three amps I had, all on Dunlavy speakers.
My greatest concern is where will I ever find a better speaker for the cost once these are old and tired.
My greatest concern is where will I ever find a better speaker for the cost once these are old and tired.