Maybe decoupling it from the floor and keeping it a few feet from a wall would help with keeping the sound contained. Controlling reflections from the rear and front may help as well. It still wouldn't completely contain it, but it would help keep the bass and highs from creeping through the walls.
I've used a chair, rack, and a custom, home built amp rack with or without foam/blankets to raise and prevent the bass from being absorbed by the floor with combo amps. It also got it closer to or at ear level and I was less likely to crank it. An amp tipped up on a short chair works really well. If you're on a budget, a heavy curtain, couch, or bed will absorb some sound from the bottom, front, or rear (depending on placement) and something like a full bookcase or shelves with various shaped materials can help diffuse the standing waves, clearing up the sound and preventing cancellation. Anything that resonates can be treated to prevent transfer, and if it rattles, it's transferring. It may be as simple as putting a pillow/stuffed animal, or pad on or in something. Since bass and low mids transfer more easily, I would treat those as vigorously as possible.
Again, it won't contain the sound 100%, but will help. I lived with neighbors on either side of me, 20 or 30 feet away, and rarely bothered them.