Cut lower frequency to help my bookshelf speakers

You are trying to get high volume from a small inefficient speaker. No matter what you do the most likely outcome is you destroy those speakers. If indeed you have not managed to do so already. When you hear the sounds you describe, in my book and in most cases the damage is done.

The "punch" you want does not come from subs. You can add all the subs you want, really good ones too, and still blow those little speakers trying to play them loud enough to get the feeling you want. What you need are Tekton Moabs or something like that. Because the punch you want is above the 80Hz or so frequency range of a sub. And even the top end range of the sub at 80 is beyond the capability of those little inefficient speakers to output at convincing volume. You are in other words trying to put legs on a snake, never a good idea.

If the search I did is right, Dynaudio Special 25 is a $5k two-way speaker with an 8" "woofer" and only 88dB sensitivity. A poor choice for anyone who wants to feel the music. Rather than chase futile dreams of being able to EQ a square peg into a round hole you might want to consider simply replace those awfully inefficient speakers with ones actually suited to what you want.

This is not only the only solution, it is also the most cost-effective, since as soon as you get a proper 98dB sensitivity speaker it will cut your amplifier power needs to a tenth, and you will be able to ditch ML for better sounding and much less expensive amplification. Win-win.

Yes high passing your speakers will make a big difference. The mini dsp is a good place to start as it is cheap. I would try 60z-80z. I think it will give you want you need as long as long as you sit 10’ or less.  Assuming that you are with stand mounts. 

You can use an analog crossover or DSP to reduce the low frequencies to your Dynaudio speakers. High pass filter set to 80Hz. or higher. I personally like analog with a steep 24 dB/octave slope, but digital high pass is much cheaper. You won't be able to get the sound of larger speakers, so don't expect that, but should clear up the bass by reducing cone excursion of your 8" drivers at higher sound levels.

Using subwoofers without a dedicated two way crossover is just a cheap and very inappropriate path into subwoofers. Not only is your woofer bottoming out but before it does it is severely distorting everything else it carries.  Either get a digital preamp that has bass management such as the DEQX Premate or get a digital crossover like a DBx Driverack and totally bypass the analog and very poor crossover in the subwoofers. You will not believe the difference.