In the beginning I used 12db/oct Butterworth, that was just shit. Crossover frequency was determined using setup CD. Probably something around 80-120hz. Moving to 24db/oct LR was a huge improvement.
But if your listening distance is only 1m or something, I don't buy the whole "you can't locate bass frequencies" shenanigans. I had a sub to the side, just off the desk and that was annoying. I had a subwoofer under my desk, severly restricting the space for my feet, and I was still able to notice it. Might also have been the vibration I could feel with my feet. I wasn't happy until I found a solution to put the subwoofers on the desk. In fact, since I needed to raise the LS50 anyway to get the tweeter up to ear height, so I've just built subwoofers that doubled as stands for the satellites.
Mind you, the LS50 are down like -3dB at 60hz, so you can't just dial down the crossover frequency. Once I got my desk subs, I did multiple tests with crossover frequency and open/closed BR ports on the LS50 and I settled on 100hz crossover with _open_ BR ports. Sounded better to me.
- I would cross subs with 24db/oct or better
- Distance and location of the sub matters, at least with short distances to the sattelites
Optimal setup would probably be a single or double bass array, if you've got the space. Or at least multiple subwoofers.