I assure you that the power supplies in your Duo are already "linear" type power supplies. The term refers to a conventional method for deriving DC voltages and current to run the audio circuit. AC from the wall goes through a transformer to alter voltage and current as needed by the circuit. The voltage that appears on the PS transformer secondaries is rectified (converted to DC but still noisy, using solid state diodes, then filtered, using capacitors or a combination of inductors and capacitors to remove noise (any AC riding in the DC). In a high end preamp like yours, the resulting DC voltages are in addition regulated. Voltage regulation adds additional smoothing. This has been done in audio for at least 80 years. Only recently we have '''Switch mode" power supplies, or SMPS. You find such SMPSs in less expensive gear but if done well, SMPSs can be very high quality. Nevertheless, audiophiles have been convinced to favor linear PSs in their higher end gear.