Regarding the house in the back... keep trying, but be careful. You don't want someone to pull a gun or something. I do think you are on to something if they won't answer the door. One thing you might want to look into is giving a private investigator a call, especially if for any reason you really start to get a vibe that it IS the guys in the house in the back... you could call a private detective agency and find out how much it would cost to get them to investigate just that house. They are very experienced and can probably get a really good read talking to the people in the house. They are expensive, so make sure you're really clear with them about what they are going to do for you and how much it will cost.
Maybe see if you can see into the house from your own house, with a pair of binoculars or something? I'd really keep an eye on that place, try and observe what's going on at different times of day, even late at night... just trying to think of anything that might get you closer to finding who has your gear. Fingers crossed for ya... oh, and wait on buying replacement gear for a week or so... you never know, this may work out and you'll get the gear back.
One last thing...about whether or not its musicians... it has to be... you said they took the cables, right? I mean, who in their right mind would grab guitar cables unless you're doing so because you know that you want to be able to plug everything in and play it? I'm sure there was a lot of other stuff worth more money than guitar cables. Just because they play guitar, or want to, doesn't mean they wouldn't steal from someone... tons of unsavory characters out there that play.
Good luck & keep us posted.