My late 60's Fender has a 2-prong, non-polarized plug (which means the 2 blades are the same size, and you can plug it into the receptacle either way), and on the back of the amp is a "polarity" switch. Putting a grounded cable on it is one of the few modifications you can make to a vintage Fender that hardcore collectors will give you a pass on - as long as it's a genuine red plug Fender cable. They don't seem to mind, because it really is dangerous the way they made them back then.
Back in the 70's when I first started playing, some of my friends who were slow-learners, wouldn't think about checking their polarity until AFTER they stepped up to their mic and got shocked in the mouth. I got bit once and learned to check it by grabbing the neck of the guitar with one hand (making sure I had good contact with the strings), and quickly brushing the back of the other hand across the windscreen of the mic. If it was backwards you got a quick,mild tingle of a shock, but it was a whole lot better than getting shocked in the mouth by what felt like the full 110v.
If they want UL to sign off on them, all modern (US) devices have to have a ground to ensure there can only be one orientation of the hot and neutral, assuming the receptacle is wired correctly (which is also a dangerous assumption). If it's a device that can safely operate ungrounded, one blade is wider than the other so you can't get the hot and neutral reversed. If it has neither of those, then it's something they've deemed poses no potential shock hazard regardless of electrical polarity and absence of earth ground.
A power conditioner can absolutely filter out noise, if everything else is kosher - Unless of course you plug your power conditioner into the wall using a 3-prong to 2-prong adapter, at which point all bets are off.