Neighbor Complaining...

If you don't have a sound level meter, buy one. (The Radio Shack models can be had for about $50.)

That can form the basis for really finding out how loud you listen and also what level bothers the neighbor. If you're a 100 dB kind of guy, you've got your work cut out for you.

However, the above suggestion about inviting the neighbor over and offering to work with him on both the volume and hours issue is a great idea. There is a fair (though certainly not absolute) chance he might try to meet you halfway.

Get a house, headphones or a louisville slugger.

Moving the speakers to the other wall will help however you can greatly reduce the sound transmission through the common wall by screwing 2"x1" timber battens @ 18" centres to the common wall with 1/4" rubber pads between the battens and the wall at the fixing points. Fix 2" fibreglass insulation floor to ceiling between the timber battens and screw/glue 2 sheets of 5/8" (16mm) dense plasterboard one on top of each other, to the timber battens leaving 1/4"-1/2" gaps at both floor and ceiling which you then seal with silicone sealant.

If all of the above good suggestions fail --buy 2 jl audio f113 subs --you will get rid of the neighbors as well as any insects that may inhabit the building :) good luck with your situation

Interesting comments about subs...I used to use my sub in order to get the neighbors to be quiet. My upstairs neighbors had a young son who would sometimes make a whole lot of noise...if he got loud, I was always able to get louder. I would say push for the eviction!

Have fun,