The GX's will take a good week or two to start settling down. Some speakers that use all metal drivers sometimes take a short while to warm up too. Some may think it's crazy, but even if the amp is already warmed up, they can take half an hour or so to come on song. Some will say that's just your ears settling into them, who knows
You'll also find that warming the 8xpd up properly before a listen will reduce any brightness too - I'd recommend keeping it on all the time, if possible.