Problem with lower frequency bass. Please advise.

I "hear" your pain. Bass is extremely complex as the room is so important. It is actually part of your "speaker" cone. No two speakers load to the room alike.

Most put the new speakers near where the older speakers were. This is almost always "wrong" for one of them. To get a good start, go to Vandersteens web page and look at the set-up instructions for speaker placement. Then, mark the practicle locations in your room.The spots should try to avoid bass resonance that makes thing BOOM. Sometimes we get used to BOOM, and don't like properly uniform bass.

After you measure your spots, move them about locations in a two foot circle. Usually, closer placement between speakes will reinforce bass if the walls aren't too close. If they are close to walls, the walls will dominate the bass boost.Don't be afraid to go "extreme" and stuff the speakers into a corner to reference worst (best?) case bass. Pay attention to the imaging, though. You want it to be balanced in the end.

My C4's and Quatro woods NEVER have sounded the same in the bass. So some of the issue is simply the bass "tone" itself. The Quatro was a warmer "heard" bass where the C4's are a visceral "felt" bass. The measured response was the same, or enough to say they both had ample bass extension.

If the DynAudio speakers you own now is like my C4 II signatures, the bass is a tighter crisper sound by nature of the speaker.

If they are on the short wall, try them on the long wall, or vice-versa. Believe me, the bass will change and then you'll know which configuration you like better...

Is everything in phase?

Try different placements in the room. If that doesn’t help your room may be too small to make some of the lower frequencies. Understand, though, I'm not an expert and your room size may be OK, but it does seem a little small. One last thing would be to see exactly how the speakers were set up in the store and compare it to your system.

Rower30 is right, most of the Dydaudio speakers I've heard have tight punchy bass but do not extend to the nether nether regions. A simple audition may not reveal this issue. Sometimes you have to live with equipment for awhile before these things become apparent.
In all honesty, I don't know if I'd worry about extension unless it's really bothering you. I went from a speaker with breathtakingly punchy bass ( Revel F50's ) to Revel Salons, and even though the Salons are far superior I still very much miss that quick punchy bass. So I guess it's a bit of a trade off, if you want speakers that extend down near 20Hz your going to lose somewhere.