FWIW, my DEQX HDP-5 preamp/processor, which is their top-of-the-line model, uses a switching power supply. Their somewhat less expensive previous top-of-the-line model, the HDP-4, uses a linear supply.
I suspect that a factor in their decision to make that change may have been a need for additional internal real estate, because the newer model incorporates a touchscreen and some other functions that were not provided in the HDP-4. But in any event I have been very pleased with the unit’s transparency, absence of coloration, and total absence of noise at any volume setting.
Charles, regarding your question my guess is that contributing factors to the continued bias toward linear supplies that seems to be evident among many designers include their own familiarity with the use of those supplies, and perhaps also an expectation that many potential customers would be similarly biased.
Best regards,
-- Al