As basals aren't actually connected to the pelvic region, being considerably higher up, they have far fewer restrictions on flat surfaces and need only consider how the back would interact with their bodies. Sweeps (the long, swooshy bit of their tails) are extraordinarily flexible, but basals are more stiff, being originally built for up-and-down motions (like dolphins). There's also the matter of Tells being a sort of head with their own brains, meaning there are issues of circulation and cushioning. A Tell isn’t about to be left on the ground, I can say that much for certain.
Bases (lower left example) act exactly like those of earth mammal, with the specifics differing a little based off of what animals were used to splice your strain of the virus- almost always felidaes and primates. Very rarely, marsupials (][Blights are occasionally called possums because of this).
Bases are continuations of the original spine, thus they are far lower and have much more trouble on most surfaces. A Vulp-en with a base would probably have their chairs made with holes in the back for their tails to hang out of, as they're thin enough for this to be an easy task.