"Burn-In" quietly?

Perhaps you can place the speakers face to face.

I have seen where some use sleeping bags ( covering ) or pillows to soften the sound.It makes sense if you care to try it....

Depends...some SS amplifiers and most, if not all, tube amplifiers require a load (like speakers) when they are powered on. They may get damaged if there isn't a load connected.

That being said, I'm in the process of breaking in a pair of new Dodd amps, and I'm using 8 ohm, 50 watt power resistors connected to the outputs of the amps when not listening to music. These act as my load.

Funny thing...when up close to the amps, I can hear the disc playing! I think it's coming from the tubes. Pretty darn wild!

Connect your speaker wires out of phase [on only ONE speaker, reverse the + and the - wire], then place the speakers, facing one another, about 1 foot apart. Same operating principle as noise canceling microphones and headphones

It is not worth the risk of burning up the amp to burn it in! I agree with Swampwalker & 1markr, some amps especially tube do not like being operated without a load. As much as it is an inconvenience to continously play music through gear for the purpose of break-in with speakers or resistors connected, it beats repair costs.