I have never had any issue in sound that I am aware of putting the pre through a conditioner. I have restricted the amps many times with conditioners and now plug mine directly into the wall.
Ditto ...almost.
On my tube hybrid pre-amp I found the PSA HC UO to mainly clean up the top end but perhaps also to improve soundstaging and imaging just slightly. It made the same type of improvement on a Plinius SA100 mk3 class A power amp, but more dramatically and also helped the bass.
I replaced the Plinius with an AKSA 100N+ which sounded flat and undynamic on the HC UO, so I plugged it straight into the wall.
Just recently, I got a PSA Duet for a friend who asked me to burn it in and give my impressions. I popped it on my current amp (AKSA LifeForce 100) just to burn it in more quickly. Surprisingly, the bass was now more punchy and dynamic, and the soundstage was much more open and natural sounding, less mechanical. The presentation was also more immediate with a quieter noisefloor, greater detail and less veiling between listener and musicians/vocalists. After burn in I moved the pre-amp off the HC UO and moved it onto the Duet with the amp (but in zone B for isolation). More of the same. Much greater immediacy, detail and clarity. Much greater improvement than I expected. I'm just leaving it this way for a little while, then I'll try moving the linear Bolder SB PS onto the Duet too (from a separate HC UO that is has been running on).
I should mention that I was running the stock Volex cords between wall and UO's, whereas I am running a Signal MagicPower cable between wall and Duet. So, some of the improvement may be due to the Signal PC. After I have the Bolder SB PS on the Duet and have become accustomed to it, I will replace the Signal cable with a Volex for a direct PC comparison.
