Behringerin Truth B1031A

I ordered Behringerin Truth B1031A studio monitors from Thomann (249€) because I was interested to see if I could use them as standalone speakers without a subwoofer for my home theater setup. Speaker elements are reasonably large (8") so I figured it might work. The frequency response goes down to 35Hz (-3dB) which is pretty nice. My first impression was very neutral and nice sound overall. The speakers even came with a calibration certificate and a measured frequency response.

After I had listened to them for a while I noticed that deep bass, below 60Hz sounded somehow unnatural. I did some measurements and noticed that there is significant distortion at 3x the base frequency. For example when playing 40Hz sine tone at 0dB (just a relative number, volume in the low linear range) there exists about -20dB tone at 120Hz (measured directly in front of the speaker). This is rather significant. For example I measure from my cheapo 6" main speakers (OR-CINEMA ZOUND M1) to have about 44dB between 40Hz and 120Hz in the same room, position etc. This sort of distortion makes deep bass sound very unnatural and at least for me, these speakers are basically unusable below 50Hz. I do not know if this is electrical problem or mechanical, but never the less it seems strange to me. It is likely that it has something to do with the design of the port because blocking the port by hand effects the amplitude of the harmonic. Now I don't like to complain too much because I really like the sound above 50-60Hz (harmonic content vanishes) and I plan on using these speakers as main speakers in my home theater setup, but I am going to need a subwoofer because I really like clean 30-60Hz and unfortunately B1031A just won't do it. The no-signal noise floor isn't the lowest either, for example my 100W home theater amplifier + OR speaker gives lower noise floor. Not a big issue, but just something I noticed. All the tests were performed using 24bit/96kHz signals (direct digital optical input, output directly from the main XLR output of the DEQ2496 to maximize SNR and minimize THD). Perhaps I could mention that the Truth B1031A is probably very good for people who are not so interested in the low frequency range and in my opinion they do produce overall very nice and clean sound. They are even pretty loud and Behringer's claim of "chest-pounding bass" is not totally exaggerated.

Spectrums of 40Hz signal I measured. Relevant information is the height of the 120Hz peak. Ignore the phase noise (peak width), it is related to finite sample length.

One a related note it would be very nice if Behringer DEQ2496 would have a volume control and a crossover for subwoofer so one wouldn't need to introduce more noise by using an external crossover and a mixer.