I think that KM shoot themselves in the foot with their "one size fits all" naming of what is really a wide product range. Printers need a System 7 with FACI etc. or similar. Anything else simply does not cut it and trying to save money on the spec is a false economy. I think KM shoot themselves in the other foot by allowing sales channels that don't really understand what printers need to sell the whole product line. The accidental shooting shooting metaphor is getting a bit stretched here, but they shoot themselves in some other bodily part by overselling the product. Printers really need to understand what it's good at and what it's weak at. But then, just about every vendor of machinery is guilty of that!
The bottom line for the OP is either:-
* You've been sold a machine without the heavy duty paper feed unit (PF602? from memory). Without this you really have no hope of getting good "registration" (I'm assuming you don't literally mean screening registration, but either F-T-B registration or sheet to sheet consistency). If this is the case, then you've either been mis-sold, or you plain chose the wrong spec machine.
* Your machine is broken.
* You haven't been trained right.
All of the above are fixable & I can assure you that on our 6501 System 7 the registration is extremely good over a wide range of stocks.
Hope that helps.
@Drawkward, you are wrong about colour and I can only assume that the KM dealer getting you samples was not up to the job. We run both machines in daily production, the Xerox with over a million clicks and the KM with about 400K (we've only had it a couple of months). The KM is just in a completely different league in terms of colour accuracy and consistency. We have several other issues with the KM, but the quality of the print is not one of them!