I have my 3801 on -15 and cant believe it doesn't go any louder
Anyway back to the question in hand , remove the skirting and add a layer of soundbloc plaster boards to the wall that backs onto their house. Either dot and dab them on using board adhesive or stud the wall out and board it!
Sticking is the easy way but if you stud it out you can add insulation to give even better soundproofing! It also creates a void which you can us to run cables in! The only down side is you'll lose a few inces off the size of your lounge!
Failing that i have a baseball bat you can borow lolol
Jon