This sounds quite a bit like my finished basement listening room. The only difference I can tell is that mine is about 8' longer on the back of the L and the foot is another 5 feet wider. I'm surprised the pipe is going mean losing as much as 1.5 feet. I have a similar situation with the main sewage pipe that drops into my basement and then exits through the concrete wall to the septic tank. This required the wall to be located about 4" off that concrete wall. The stud wall that runs down the other concrete wall is held off 2" so as to prevent moisture wicking into the insulation from the concrete. I do wish I had hung a double thickness of drywall but it's not really a big deal.
I have come to find that we who have L shaped rooms have a somewhat different room in which to work as those with more rectangular setups. But that doesn't mean we can't achieve just as good results, it usually just takes more time experimenting with placement and acoustic treatments. For instance, I have found that my system fits the room and sounds best when set up completely in the toe and playing back towards the heel. Then to simulate the open side which is the top of the L I have added absorptive panels down the sole of the foot which is the right hand wall from the listening position. This and many bass traps are working very well but only after I got the speakers in the best location.
I'm sure Rives and others can give you better advice on how to construct the room. My comments are meant to suggest that you can still get great results with location and treatments once the room is finished so don't get to obsessed and make yourself crazy. Good luck and enjoy the process!
I have come to find that we who have L shaped rooms have a somewhat different room in which to work as those with more rectangular setups. But that doesn't mean we can't achieve just as good results, it usually just takes more time experimenting with placement and acoustic treatments. For instance, I have found that my system fits the room and sounds best when set up completely in the toe and playing back towards the heel. Then to simulate the open side which is the top of the L I have added absorptive panels down the sole of the foot which is the right hand wall from the listening position. This and many bass traps are working very well but only after I got the speakers in the best location.
I'm sure Rives and others can give you better advice on how to construct the room. My comments are meant to suggest that you can still get great results with location and treatments once the room is finished so don't get to obsessed and make yourself crazy. Good luck and enjoy the process!