If you have a line conditioner, does noise matter?

I have a PS Audio Power Plant Premier, a DIY parallel AC filter, RC filters inside my preamp and EQ, snubber caps on all tube heaters, ferrites on the heater and B+ lines, and my preamp uses DC for the signal tube heaters. Every single additional noise reduction step decreased background noise and improved the sound, and I started with the PPP as the first one! They are additive.

My understanding of these various line conditioners is that they control spikes and brown outs to keep the volts at 120 +-. Shouldn't this take care of the ac "noise" on the lines allowing only the music to flow through unimpeded?

Conditioning the AC power can be done in 2 ways - line conditioning using passive devices, which is what you are referring to & AC power regeneration which is what units like PS Audio Power Plant Premier & others do.
With a (passive) line conditioner - like you are referring to - no new information can be created. I.E. all that a passive line conditioner can do is to supress the grunge on the AC line. The grunge is reduced but not eliminated. So, some residue grunge still gets into the audio gear. The better your audio equipment & the more trained your ears are, the more likely you are to hear the effects of this grunge (like your friend).
It is very difficult for passive line conditioners to control brown-outs. For example, I have a RGPC 400 Mk1 & it can control brown-outs that last for 1/2 second or less. Anything more, it'll go down as the AC power goes down. All this makes sense - there's only so much charge that an AC power capacitor can hold & supply to power hungry electronics.

re. "the music to flow unimpeded" - this is a trade-off. The more you reduce the grunge on the AC power line, the more stages of filtering you need. More stages of filtering hinders the unimpeded flow of music - all AC power line filtering components have parasitic resistances that do not allow all the current to be delivered to the electronics.
Plus, all AC power line filtering components have their own sonic signature which adversely affects the final sound you hear.

So, to your question:

If you have a line conditioner, does noise matter?

yes, it does.
I feel that it matters less (but it still matters) when you have AC power regeneration.

It would be wacky expensive, but couldn't a motor driven Variac be used to automatically control output voltage to 120, or whatever desired?

03-10-11: Magfan
It would be wacky expensive, but couldn't a motor driven Variac be used to automatically control output voltage to 120, or whatever desired?

Not expensive at all. I have this one (not in my audio system), which I once had occasion to use experimentally. Only $119. Just one problem, though -- it is absurdly noisy every time it decides to run the motor, which was every few minutes at my location.

Best regards,
-- Al