Romy, what do you mean by "the negative curving is longer to engage"? I picture a platter that has been machined to the negative trace of the LP face but without any means to "engage" the LP unless there are further stipultions, such as weights, clamps or vacuum; in other words, the negative trace itself is just a "mirror" resting place where the disc sits +/- , according to how flat it happens to be, unless and until the disc is made to conform and sit perfectly tight by other means. Of course Micro did have high-tech vaccuum TTs available, too, although I know nothing about them, nor do I know anything about their propritary clamps, if they had them.
It appears however that I am the oddball, with the vacuum, since the vast bulk of TTs are sold and used without any means, let alone any effective means, to couple the LP to the platter, negative curve or no, and most people who use the clamps believe they are just flattening the warped LP with them. While many people want to think they are coupled, most of that lot basically does nothing active to ensure that it happens.
If anyone following along has used low-pressure vacuum for a while and then gone away from it, please speak up and offer some intelligent reasons for your switch. Although Lloyd Walker makes some very interesting observations during his clamp versus vacuum arguments, the "tap test" and subsequent listening comparisons have convinced me of the generic fact that well-done coupling is very beneficial. In my comparisons I hear no advantages in loose discs. Needless to say I am curious to hear the Walker, although it could be pretty scary, given the ancillary equipment they all seem to use. Even so, as I laid out in my "Balls to Shop" thread, there might still be some There there.