Shaggy, the housing of the MK 8 was revised about ten years ago to leave somewhat wider spacing between the slats. Too much "bombproof"-ness can restrict the low-frequency response of a capsule at times, as it turns out.
With Schoeps capsules, since no one else does repair work on them (or should do so, at least), it's very easy to find out when it was at the factory last--you can send a note to mailbox@schoeps.de including the serial number of your capsule, and they can look it up for you. If your MK 6 is one of the early Colette models then I would recommend sending it back via Redding Audio in Connecticut. In fact if it's been more than 15 years since it was serviced (which would include many capsules that aren't among the very earliest), I'd recommend it.
The clearest symptom of trouble in an older three-pattern capsule is if the omni pattern isn't so omni any more. In a microphone that needs the gaskets replaced, the figure-8 position nearly always works the best of the three patterns, unless the mechanism itself is broken. By the way, these older capsules should always be stored in the figure-8 setting, which often allows the gasket material to spring back somewhat. But best of all is to have the capsule serviced--they've got a new type of synthetic rubber now which lasts a lot longer.
The two cardioid settings are identical--there's no difference in which way things face.
The polar response of the figure-8 setting is in fact highly symmetrical--but keep in mind that 16 kHz, which is one of the standard "spot" frequencies for polar diagrams, is the very limit of this capsule's response. The pressure gradient is just too small at such short wavelengths to move the membrane with as much force as it has at lower frequencies, and there's no pressure response to compensate (as a cardioid or supercardioid would have). So the response rolls off at that point in the figure-8 setting. But the pattern remains basically that of a figure-8, unlike some other microphones in which it narrows considerably or else takes on strange shapes with abrupt peaks and dips in the response.
--best regards