Tripp Lite and others make an active unit that stay at 120v with 89v-140v inputs by auto selecting taps on a transformer. The units also have EMI and RFI filtering. Wouldn't these be preferable to filiter only units? They are very reasonably priced - especially compared to some of the filter only unit. Is there a downside? (Lastly - it seems to me it might add some protection from lighting?)
1. Depends. Most modern equipment will operate without problem and without compromise over a window of input voltages. If your AC varies only within this window, you have (a) no need for an AVR and (b) no need to introduce an actively-adjusting device into the circuitry. Why bother?
2. As others have pointed out: filtering, surge protection and voltage regulation (to say nothing of UPS) are different functions which can be used independantly or in various combinations. None is panacea nor is the blind insertion of all. If you have an AC problem, solve it. Otherwise, pay attention to the music.
I lean toward Kal's opinion. I've tried power conditioning and noticed no improvement of any kind and when used with my amp I actually noticed degradation. While this is certainly no basis to form an absolute opinion, it certainly suggests the wisdom in trying before you buy.
I would be willing to bet for the same money as a decent power conditioner you could hire an electrician to run you a dedicated circuit to your audio stuff.
Monty has it right. Dedicated lines make more difference than any of those conditioners. I wouldn't use one on a power amp anyway. I would be concerned about current limiting. I know how difficult it is to build a regulated power supply with no current limiting.
I have dedicated 20A lines in my listening room and use a RSA Duke on my power amps and a RSA Haley on my front end gear. They're not "active" but they improve the sound quality and prevent the digital equipment from effecting the other components.