Joule Electra LA -150 ii/ 200 and impedance issues

Al, As always, your insights are extremely valuable and in the past have prevented me from bad decisions, like biamping on single terminals:)....but are you saying that, if a preamps outputs are paralleled, which yoy say is common, bi-amping may cause issues without our knowing it because the combined impedance load is too low for the preamp? Isnt it conventional wisdom that the ratio should be 10:1 of impedance output:input? Maybe this isnt often achieved and maybe bi-amping may adversely affect frequency response and, on the whole, be deleterious instead of additive? Do I misundertand you? This is surely a fundamental question?

So, before biamping, this calculation and these data should always be done and obtained, or you could end up with worse soound at great expense? WoW!?

Interesting that, in all the various bi-amping posts, I've never come across this concept. Am I blind, is it less practically important or are others equally unaware of it?
You may have trouble answering this question:)

Jud Barber says, in response to my e-mail, that his preamps are designed to work best with impedance 25K ohms and above. So I should be safe. thanks for your help, Al and Bifwynne