upgrade path from my current speakers? (Dynaudio Special Forty)

Funny you mentioned the sound of the special 40. I went through the exact same scenario. Found a good deal on a pair of Heritage speakers. I had  them awhile but they just did not engage me like my 40’s I sold th Heritage and bought an another pair of 40’s. I keep several speaker and amps as I like to change up sometimes. My PassLabs and my 40’s are my best combo.

Alone, I own both dyn 40’s and kefls50 metas. The metas are world class IMO. However, the dyns are a better speaker, PROVIDED, you have some horsepower to drive them. The more juice you give them, the better they sound. I have several amps, like Accuphase, McIntosh, hegal. The 40’s sound the best with my Pass 250.

A sleeper speaker is the Goldenear BRX. 

Regarding Buchardts, I have owned the s400 and now own the 400SE. There is a clear difference in the sound between the two. The 400 is a warmish-sounding speaker, whereas the 400SE is more neutral. The 400 has a laid-back treble the SE is more forward in the treble. That is the most notable difference The Speical 40 IMO is better sounding than both. The 40 is more energetic and punchy in the bass. Having said that, the Buchardts are very good sounding for the money. They do not need as much power to sound their best as the 40's.