Between the 7 subwoofer systems in my home and 5 systems in my office, I have found placement to be further down the list of changes which affect localization. My office subwoofer is several feet behind and to the left of me and it is harder to integrate than my home systems. I start by level matching the subs and mains. However, the most important adjustment I have found to help is to get the subwoofer and mains integration correct at the crossover point by using the distance setting. Sealed subs located in the front might need to be a couple feet further than their actual measured distance, ported subs can need around 5 ft, and horns might need up to 17 ft extra. This is just a generalization based on a lot of subs I have measured. You might actually need to reduce the distance setting for some circumstances and location. I like use use the RTA feature in REW and adjust the distance until I get the smoothest response around the crossover frequency.
I also play with different crossover points and different slopes on the crossover simultaneously with the distance setting. Most can't change slopes with a receiver. I can change the low pass slope on the crossover of the sub between 12, 24, 36, and 48 db/octave and I can also change the high pass on the mains to different slopes. By varying the crossover frequency, crossover slope, and distance setting I am able to best integrate the subwoofer with the mains.
The next thing I do is EQ the sub(s). A peak in the midbass or inductance hump will cause one to localize the sub. A smoother frequency response
Even though I start with the subs and mains level matched, it doesn't have as much impact on localization as the distance setting or EQ. Once the distance setting is dialed in and the frequency response is EQ'd, I then raise the sub level by around 3-6 dB. If the sub and mains are well integrated and the subs are EQ'd you can run the subs hot and still not localize them. There are still situations where room placement will limit the variation in SPL between the sub and mains if you don't want localization.
In summary, the following can be adjusted to help with localization:
- Crossover frequency
- Crossover slope
- Distance Setting (Use phase if distance isn't available)
- Equalization of subwoofer
- SPL difference between mains and sub
- Subwoofer location
- Upgrade your entire system