I'm not 100% as I don't know how much room is behind the bow speakers of the 01 LS (maybe someone can speak up) but I don't think you need the slims? The only problem place I can see is up in the bow. There is more than enough room for the rest of the cockpits.

I have an outback 07 and there was plenty of room for the regulars all the way around!

It's a great upgrade and you won't be dissappointed