Onix A-120MKII Gets hot fast.

How far apart should they be?

what kind of speaker wire are you using?

6-8 inches is sufficient. Just not on top of each other....

Speaker wire is just some old bi-wire no name stuff with thicks and thins for each side. Rather stiff. I have thought these old wires might be the cause, too much resistance but I don't know. Never a problem with the old Rotel so I didn't buy new wires to see. The amp does sound bass heavy at lower volume. Kicks butt when I turn it up.

Looking at a picture of the integrated, it appears that there are no external heat sinks and only a small slotted area on the top of the case for convective cooling. If your stacked equipment covers those slots then you are almost certainly going to have a cooling problem. I also note that the amp does not double down from 8 - 4 ohms, and that even though the nominal impedance is 8 ohms, the minimum is spec'ed at 4 ohms. So my guess is that the amp may be struggling a little to make the necessary power into the 4 ohm portion of speaker's impedance which causes it to run a little hot and then the stacking of the cases is making it worse. And of course the component on top is also releasing heat, some of which is radiating downward to the amp.