I'm not too sure about the effects on soundstage as I've never used anything other than stock pads myself. I know that the HD800S has a dead-zone where the soundstage is perceptibly wider, so that may be something to consider with 3rd-party pads as they might alter that angle. Don't take my word for it though.

As for tightness, is your headband adjustment all the way in (the tightest setting)?