Rega's now legendary head electronics engineer Terry Bateman spent 10 years researching the concept of a CD player using valves in the output amplifier. Valves have been widely used in musical instrument and recording amplification from the 1950's to the present day, therefore it made great sense to develop a valve based CD player to match that of the signal chain found in such applications.
This approach kept the circuit straightforward. This is mainly because musical instrument amplification is not excessively hindered by some of the folklore surrounding certain aspects of valve audio design. He took the approach of combining the technical and sonic attributes of valves in a differential instrumentation amplifier circuit driven from differential output DAC with a passive LC filter.
The Rega Isis is available in both solid state and tube versions. The solid state version is a little quieter while the valve version has more texture and sense of presence. Visually the tube version has four tubes on the top deck to the right of the transport lid.
Rega Isis Reference CD Player / DAC Technical Specifications