I don't see why it wouldn't be and this may be a stupid question.
As you can see in
THIS picture my speakers are not at the same level. Also, the left is on an enclosed shelf. It doesn't affect the Advent in their now too badly as it has no port. However, I'm planning on building a set of
Ascendant Audio Arbiter TM's in which I will use a port. I would like to kill two birds with one stone and place the left speaker in a better position (ear level) and keep it out of an enclosed shelf.
The easiest way to do this would be to set it on top of the subwoofer, which is roughly the same height as the table the right speaker is sitting on. Am I thinking too hard here or is there no problem in doing this?