Hi Frafra,
Your cube face tiles are 1242 pixels. Cube face compression is set for 100% or no compression.
There are two elements that cause this issue.
Cube face tile size controls the image quality.
Cube face tile compression controls the tile compression or size like 2mb
You are using a 6000 w x 3000 h pixel Tif (8 or 16bit ?)
Calculating best tile size = stitched pano Width dimension x Pi or 6000 x 3.1415 (pi) = 1909.91 or 1910 pixels (compared to your 1242 pixel)
EDIT Correction 
-> Yes Divide 6000 / 3.1415 (pi) = 1909.91 Got the answer correct.. math process wrong.
Suggest you first rebuild your Cube face size starting with 1910. This will probably generate too large output.. like 10 - 20mb with great ceiling area.
Slowly reduce the cube face value say to 1810... Check the ceiling quality for color banding artifacts.
Again 1710 Check the ceiling quality for color banding artifacts
Again 1210 Check the ceiling quality for color banding artifacts - IF YOU SEE artifacts then test again at using 1510.
Once you are satisfied with no color banding artifacts... Now try changing the Cube face tile COMPRESSION 100% down to 80% (70,60,etc) and check for artifacts.
You can also change compression for each tile (70% left, 70 back, 70 right, 70 front, 40 top, 30% bottom). These values will override (replace) the general 100% for all tiles)
We use 6000 x 3000 16bit images at 140 mb. We sucessfully get a final 1,5 - 2,0 MB Flash .swf that is sharp.
Hope this helps.
