In my first post I did mention that I view your file in Acro reader and it looks exactly like both PNG files I showed above. Now I don't have the luxury of having CS4/5 but do have CS3 and when I opened the file yesterday they had no mask in place and were in fact they were even grouped.
Should tell you I worked for a health board in Scotland and I am in and out of both Illy & D pro 6 daily and use Acro Pro for proofing and do not have any problems with the software. There is a known bug in Acrobat at high levels of zoom but this is not your problem yours were clipping of lettering in each of the 2 designs. This maybe the fault of Xara's export filter as it looks like you used an envelope to distort your text but I tried that and couldn't replicate your effect. So I have tried everything that you have asked but I can't get the same as you but I don't think it is a bug in Adobe's reader 10.