I have never heard the Cello tape machines, but I had some experience with Cello units in the 90's, as I owned several Cello systems at that time - first a Preamplifier Palette and Duet amplifier, later an Audio Palette and Etude amplifiers. The electronic modules used in the more expensive units (as well as in the Apogee based reference DAC) had definitively a "flavor", adding body and some sweetness to sound, making it very nice sounding. But the cheapest units, based in Analog Devices ICs were cleaner sounding - the Preamplifier Palette and Duet were a fantastic match for Quad ESL63, making a very neutral system.
Another characteristic of the Cello units was that they were difficult to match with other brands.