Here's my take on subs with 2 channel hifi. I don't use one. Here's why; I prefer sins of omission to sins of commission. WIth a sub in a 2 channel system, you will see benefits with some tracks, but with other tracks you will absolutely hear bass issues that degrade the sound. Without a sub there are no issues like this. You simply might not get the last iota of deep bass. And with Dynaudio speakers, it really is just that last iota of bass since they are bass champs.
If you do get a sub, get one with built in DSP room correction. Subs like that have the ability to "mic" the room and compensate for room acoustics. Without that you will get lumpy bass guaranteed.
Finally, I would say that there's a big difference in how different amps deliver bass to the speakers. Some amps are much better at it than others. I'd focus on the right amp first.