AC/DC
AC means Alternating Current, DC means Direct Current.
That means that with DC, the current runs only one way (from + to -), but with AC the direction of the current alternates. It goes first back, then forth then back then forth. It does this 50 or 60 times a second (depending on where in the world you live). This is the cause to all that hum you get when you record...
Now, when it comes to power supplies and noise:
The current you have in the wall as AC. Electronics run on DC. Therefore you need conversion. With cheap power supplies this is often done badly, and the DC power that comes out are noisy, it has a lot of ripples in it, remains of the AC. Some units (mainly Boss ones) rely on the current being clean, and therefore starts generating a lot of hum with a bad power supply. Therefore you need good DC power supplies.
This however is NOT the case with AC power supplies. AC units does their own conversion, and hum and ripples on the power will in general NOT generate hum on the output of these units. Therefore, switching power supply will most likely not help you in this case.
Also, the fact that you even consider reversing polarities make me very worried. If your BOSS and Alesis power supplies are AC, then there should not be any notes on the about polarity. And if there isn't any such notes, why did you think you needed to reverse the polarity?
My Tech Support instincts tell me there is something wrong here...
In short: Don't do it. Not yet at least. I need more info to be sure that you are not gonna burn something.