subwoofer for music?

The reason for your confusion about the low end extension of small-woofer systems is that the frequency response specs rarely indicate how much acoustic output they can support at those lower frequencies. For example, a 6.5" driver and a 10-12" driver may each play a 30Hz tone but the latter can play it much louder. Note that the drivers in the subs you list are larger than 6.5".

Also, you should consider using a much higher crossover than 35Hz for several reasons.
1. Crossover roll-offs are gradual. A 35Hz crossover will have minimal attenuation one octave down and, with your speakers, have little effect.
2. Raising the crossover will transfer more of the LF power demands to the more able sub, reducing stress and IMD in your main speakers.
3. Raising the crossover and getting a sub with decent EQ will make placement easier and integration smoother.

However, how do you intend to connect and crossover the subs?

01-02-14: Bob_reynolds
Will 2 subs help stereo imaging?
I don't think there is any notion of imaging at bass frequencies.

Really? Well I have a notion of it because I hear the imaging pop into 3D when I get subwoofers integrated properly. It's not that subs image all by themselves; it's what they do along with the mains. There's something about reproducing strong fundamentals and room energy that provides a better foundation for the overtones handled by the L-R speakers, and that improves imaging and soundstage.

BTW, YES, you can get strong, deep bass from 6-1/2" woofers. I have a pair of Mirage M5si's, which have two 6-1/2" woofers, one facing forward and one back. The cabinets are large--50"h x 18"w x 8"d, or about 7,000 cu. inches. You can't make a categorical statement based on driver diameter. Stiffness and weight of the cone plus driver excursion (how much air does it displace?), and cabinet volume are all part of the equation. Fed enough power, my Mirages can make deep, loud bass.

With Stereophile's test CD I have verified that my 6-1/2" woofers take me flat down to 29 Hz and are still usable/audible to 26 Hz.

If you look into the OP's speakers, you'd find that each column is packing at least 3 woofers--one forward facing and additional internal ones making an isobaric arrangement. Since the speaker is discontinued it's harder to determine the exact alignment, but they either fire downward into the plinth (as in other Neat models) or the energy exits via the rear port. At any rate, isobaric alignments are capable of quite a bit of bass, and at good volume.

To the OP: I recommend a pair of JL Fathom F110s or F112s, whichever your budget can allow. Integrating a single sub can be full of compromises; integrating a matched pair is relatively easy and does more to improve imaging as well. It also doubles the low bass they can reproduce. I'd go for a crossover frequency of 40 Hz.