Lightroom :: Softproof In Photoshop On External Display Not Matching

When I soft proof an image in Lightroom (5) and in Photoshop (CS6) on my external display there is a noticeable difference between the two proofs.
 
This is the case both with "simulate paper & ink" on and off (with matching settings in both applications) and with a variety of different profiles being used. On my laptop screen the proofs always match (between Photoshop and Lightroom).
 
On the external display with soft proofing off there is an exact match between images in LR and PS. With soft proofing on saturated areas of images (particularly reds, greens and yellows) become noticeably less saturated and blocked up in LR as compared to PS. Subdued colours appear to match. It's almost like they are both using the same profile but Lightroom is squashing it into a smaller gamut. I'm fairly sure PS is correct and LR is the one with something going wrong.
 
I'd like to sort it as I'd prefer to soft proof in Lightroom as that's where I print from. I've read up a lot on colour management and can't find a solution.
 
(I'm on a Macbook pro running 10.8.4 and the external display is an NEC Spectraview 271 (euro version of PA271W), the display is calibrated and profiled with Spectraview 5, with an i1 Display Pro. Working space in PS is Prophoto RGB and it's set to preserve embedded profiles. I've checked with various test images but mostly been using Bill Atkinon's printer test image. I set up the image in LR and PS to match on screen position and with the same colour (white) background, I can then flick back and forth and there should be no change but there is and it's very obvious.)