We have a very similar problem. We're a house of music students, living in a student house - we HAVE to practice for our course. We keep it right down, don't even plug the electric drum kit in, and we keep all the amps low enough to comfortably sing over the top without breaking out the microphones. They still complain! We never rehearse after 7pm so it wont disturb them watching TV in the evenings or whatever old people do that's so important... But every time, 15 minutes in the doorbell will be ringing and they'll be telling us to keep it down. We always apologise and try to turn it down as much as possible but I get the impression it's not good enough. We pass the old couple in the street sometimes and we smile and say hello like good neighbours... But they just give us a dirty look and blank us. We've tried organising with them when we can rehearse so it wont bother them, we'd agree on a date but then on the day they'll come round in the morning and say "actually, can you not practice?" Well... We kinda have to at some point! I don't know what to do with these people, you might be able to have words with your neighbours and sort it out but some people are just bitter and can't think outside of their own little world.
You might just have to fork out for rehearsal rooms, bro!