This depends on your definition of "best placement." I have dual subs under my M105's as well in my near field setup. I can hear where my subs are located with an 80 Hz crossover when listening to music. Having the subs anywhere other than under/inline with the mains in my near field setup sounds worse, because the lower bass is not coming from the same space as the mid/upper bass, and I can hear it. Measurements mean nothing here, because it's a subjective human experience. It's not just frequency, it's location as well. I think that subs that are balanced in line with the stereo image sound better and more natural when listening to music, even if you could get the frequency response to measure "better" somewhere else in the room.
This is very true. And this is the problem of integration subwoofers with a small standmount speaker. This m105 has one of the best 5 inch drivers one can buy, but its not the best below 150 Hz.
The only way to gain overall soundquality with this m105 speakers, combining them with two subwoofers, is to have the crossover between 100-250 Hz ( both LP and HP ) . To do this, you need:
1. DIY two subwoofers as a 60 - 70 cm high stand below each m105. Each subwooferdriver will then be close to each m105 woofer. The subwoofer drivers should be placed at the top of each stand, pointing forward. Two 6 1/2 or 8 inch subwoofers can be used in each stand.
2. The two subwoofer stands should be connected in stereo, using both LP and HP crossover.
3. The m105 bass port should be plugged.
With this approach, you get very near the bigger Revel 208 speakers using the m105 bassdriver only as a midrange - as it was ment to be, in a threeway.
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Edit:
However - theres often a certain magic in the stereoimage of a small standmount speaker , and If you want to stick with the m105 as a freestanding speaker with ordinary stands, one can gain massive in bass quality If the speakers are optimaly integrated in the room for best perceived bass pitch - this is done by ear.
Here is how to do it: