The COBRA experiment aims to search for the neutrinoless double beta decay by using CdZnTe (CZT) semiconductor detectors. A demonstrator setup, comprising 64 CZT detectors with a volume of 1 cm(3) each, was operated at the underground Laboratori Nazionali del Gran Sasso (LNGS). The COBRA collaboration has recently upgraded the demonstrator to the extended demonstrator (XDEM). An array of nine large volume 2 x 2 x 1.5 cm(3) CZT room temperature semiconductor detectors with a total mass of 315.1 g was installed to increase the half-life sensitivity. The innovative detector design and improved detector handling procedures during commissioning allow for an improved energy resolution of the detectors while the most prominent backgrounds that were identified during the demonstrator operation are expected to be reduced significantly. This article gives an overview of the new hardware components and the background present in the setup. Furthermore, the energy and position reconstruction is described in detail, including the correction of electron trapping. All functional detectors demonstrated a good stability in the long-term operation. The analysis of the commissioning data confirms the excellent energy resolution of the detectors, as well as an improvement of the background level by more than one order of magnitude compared to the previous demonstrator setup.