Not sure if others have run into this, but my first XPAN ( a model 2) underexposed by about a stop with exposures longer than 1/4 second. Speeds seemed (sounded) OK, but it was the metering that seemed to be the problem. When I metered with a handheld unit (after noticing this) speeds in fact ran true. I've recently gotten another XPAN2 (I had to sell the first one, and regretted it immediately) but have not used it in the same low light situations yet. I will absolutely bracket when I first do to see how it does.
A wonderful camera, with excellent lenses that produce stunning negatives. ENJOY!!