Just to add to what others have said, the x400 printers have a series of onboard sensors that allow for built in calibration but not profiling of papers. The on-board calibration process allows for consistency over time and across several printers. So the on-board calibration procedure is great for a big shop with 50 x400 printers as it ensures perfect consistency over time and across all the pritners so that one profile can be used with all of them.
The lack of spectro hardware means you'll need to profile externally which is easy and fast to do with something inexpensive like an i1Pro2. External profiling has its advantages - cost savings and consistency of profiles across a variety of devices being the big two. Buying a bunch of inexpensive printers and 1 spectro makes sense for most people, but some high volume press proofing shops like to have the built-in spectro that's controlled by the RIP and that's where the spectro option with the 6450 comes in. Does that make sense?
