Personally, wouldn't rush to blame your equipment. In my experience, a relatively minor (yet admittedly alarming) pop or such is fairly common on powering down high current gear. Order matters, and that is why you should turn off the equipment closest to the speakers first (and on last) precisely to combat amplification of power bleeding artifacts to the speakers from gear up the chain. If the amps off, dont matter what the preamp does when you turn it off, you get the idea. Put differently, it's common enough that it is an accepted rule of thumb. And unless your amp is really dramatic, wouldn't let it concern you. (And by dramatic, you'll know it when you hear it. For example, I know that Bryston amps and likely all amps utilize a muting switch on power down that engages immediately to break the feed to the speaker taps so that, as power dissipates from wherever it's stored in there, you don't have to listen to it, you might get a pop, but it saves you from the worst of it. Well, apparently some time near on 20 years ago, a batch of 4b-st's left the factory with muting switches that were known to fail, and one of said amps ended up in my living room. Sans muting switch, turning the thing off kinda sounded like someone very large and cross was tearing the speakers in half. Successfully, mind you, not just trying, either. No bueno. Bryston, of course being Bryston, fixed it immediately no questions asked but, just saying, when that type of issue rises to problem levels, it aint subtle).
As for the hum, sounds very much like a classic ground loop. In fact, unless it is only in one channel in which case I would indeed look to the gear my bets definitely on a ground loop hum. To start, the most common source of a ground loop is a cable feed: you have cable TV plugged into anything vaguely associated with the speakers? If so, thats it. You could theoretically get the cable company to reground stuff, or you could put a form of filter in there to hack off the hash. Jensen Transformers (google will take you right there) makes a whole variety of options for this. They work like a dream and I use one (the Cable TV RF Isolator).
If no cable TV, could still be a ground loop. Ive found that they can be insidious little beasties, and entirely beyond my comprehension. Changing the slightest thing can cause one to pop up. For example, I have likely over a dozen items interconnected throughout my system and its, generally, dead silent. One day, I swapped out the hard drive feeding the music server replacing a powered 2TB jobbie with a 1TB raid powered entirely off of the firewire. Removing that one plug from the system earned me a ferocious ground loop hum. Why? No f-ing clue, but thats the sole thing that changed. I tried rewiring everything (things are fed from three different outlets splitting or moving things is one way that sometimes works). No love. I tried lifting the grounds on single and then various permutations of groups of gear (your ground is the third prong on the electrical plug. Lifting the ground is disabling this third prong, most easily, by using one of those 3 into 2-prong adapters. In the right spot, this can also lick a ground loop hum although its generally not the best long-term plan, those things are grounded for a reason. But a perfectly viable diagnostic tool.) No love. Then I remembered that I had one of those Jensen jobbies lying around from before we moved, and that did it. But, lesson learned, changing anything can change your grounding pattern and earn you noise. In all events, a real common problem, just search ground loop here and youll get more info than you bargained for. Not saying thats your issue, but something Id definitely consider before racing around trying to fix stuff that might not be broke.