I'm having what looks like the same problem, except display calibration doesn't come into it.
If I print an image (dng with adjustments in Develop) directly from LR, then export to jpg, import that file and print it, the second print is duller than the first.
I've tried all three standard colour spaces. Jpg quality is max, resize and sharpening are off. The printer, paper and all print settings are identical. Every time, the print from jpg is duller than the original.
This is LR CC, but the relevant machine has had no internet access for the last 5-6 weeks so if there's been an update during that time it's not been applied.
Ashley, this sounds like a colour management issue - probably a profile issue.
The second image - viewed on the web - how are you viewing it? What browser? And how is it posted on the web - I mean, what site - e.g. flickr or whatever? Some browsers (especially Internet Explorer) are not properly colour-managed.
What happens if you re-import that jpeg into Lightroom? How does it look compared to the original?
Robin: I've replied to your other thread at http://forum.luminous-landscape.com/index.php?topic=100799.0 .
Below are screen shots of one of my photos in Lightroom and then posted on the web.
Check that the web browser is color managed, just because you calibrated the display, non color managed app's don't understand them so they will not preview and match LR which is color managed. You can check the browser: