The SolydX (Xfce) and SolydK (KDE) distributions are based on Debian Testing, with lots of Linux Mint goodies added in. They actually started as one users personal project in the Linux Mint forums to develop a KDE desktop for LMDE.
It didn’t take long before others in the forums were helping with the project, and when it became clear that there was not going to be an “official” LMDE KDE distribution, this became a real distribution project. It was expanded to include an Xfce version as well, and it has gotten continuously better since then.