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For planetary imagery a DOB is difficult and will need high magnifacation to get a good result. You need to find a way of getting the CCD element closer to the focal point to acheive focus, I have only used another web cam and managed to get only 1mm of travel (lucky me) but I expect with such a fine tolerance it will be difficult al-be-it not impossible. I have no XP on the SPC 900.
It is strange though that the barlow also experience the same problem, I usually find a lot more available focus with my barlow but I have only tested mine, I though have difficulty tracking the DOB for smaller planets and have only succeded in non barlow shots as the planet is difficult to align.
If you have a collapsable DOB, just collapse it a bit at a time, if not you can try to re-colimnate the primary mirror moving it forward. I did this as well and got an extra 5mm.
On another note you may get a bit of noise around the planet which is normal, this is why images are processed to remove this noise that could look like an out of focus picture.
Finally if the above doesn't work low profile focuser or cutting the tube (argh) maybe the only way.
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