Parasound Halo A21 - Power conditioner?

I owned an A21 and also have the schematics. There is a minimal amount of power supply bypassing in the power supply to absorb line tranients, but nowhere near as much as in a good line conditioner like the PS Audio or the Furman (and no MOVs or other SS protection).

Irrespective of any sonic effects, I would always use a line conditioner on an expensive amplifier simply to prolong its life. High voltage line transients can destroy solid state devices in a few nanoseconds, and they can get past the input transformers if they are high enough in frequency.

Re Halo Dealer

I use a Furman PF-Pro R power factor correcting line conditioner (which also has very good surge and transient protection) on my JC1 monoblocks and they have never sounded better. This device has capacitive energy storage which improves (lowers) line impedance from the wall plugs so that audio transients and current surges are enhanced. Also used them with the A21.

And if you think you can anticipate when a nanosecond high voltage spike is going to enter your amps, good luck. It's not always during a lightning storm. This electrical crap can be generated within your home when arcing contacts from your refrigerator, hair dryers, and air conditioners turn on and off.