0dB isn't arbitrary since it's a clearly defined value and pretty much implies that the original audio is left untouched. The old values, on the other hand, was completely arbitrary because I can't even tell you what it was - it was using an unspecified mix mode, an unspecified sample pre-amp and was thus completely random. The new behaviour has a clearly defined volume. As I said, it might be a good idea to base it on the currently edited module's volume, or it might be a bad idea. I'll just go after the general opinion if people think that would be a sensible solution (I personally prefer the 0dB preview at the moment).
By the way, please report bugs on the issue tracker the next time - the forums aren't supposed to be abused as a bug tracker anymore.