A tunable far-infrared difference frequency spectrometer has been used to examine the fully protonated form of malonaldehyde in the region near the ground-state tunneling frequency (21 cm-1). An extremely dense and complex spectrum is observed in which the strongest features have been assigned as pure rotational lines involving high values of J and K-1. These transitions, which occur within the individual rotational manifolds of the two halves of the ground-state tunneling doublet, have been analyzed simultaneously with existing microwave data for this species. The value of the ground-state tunneling splitting, determinable indirectly from analysis of vibration-rotation interactions, is 21.584 76(17) cm-1, and is in close agreement with that similarly obtained in previous microwave work. A thorough treatment of the centrifugal distortion in this system significantly extends the range of rotational states whose energies may be reliably calculated, and should therefore be valuable in the future direct measurement of the tunneling frequency. Aspects of the far-infrared spectrum of this species, and of the indirect method of determining the tunneling splitting, are discussed.