If I were to try to decipher the root causes of “this new hifi sound”, I’d say it has nothing to do with intentional frequency dips or peaks. Instead I see a few factors:
1. Advancements in speaker driver technologies and engineering, as well as in improvements in crossover designs and implementations that altogether in a speaker design reduce “breakup” or distortion to deliver a cleaner sound and more articulate imaging. Borresen Acoustics is a great example of this.
2. As much as I hate to say it, the Class D revolution, accompanied by other Class A and A/B solid state amplifiers that are providing lower distortion and better control to the speakers, which are also contributing to improved imaging and performance. I don’t believe most Class D is there yet, but at least not all of it is painful to the ears and the design is becoming more refined and musical by the day. Aavik, AGD Productions, Atma-Sphere and Bel Canto are all great examples of this. On the other hand, Class A and AB are also improving with speed and clarity while continuing to balance musicality. Westminster Labs, Thrax, T+A amongst many others are pushing the boundaries here.
3. Digital sources continue to improve with lower noise floor while keeping the full integrity of the digital signal to deliver detail with effortlessness thanks to well designed power supplies, improved isolation, and clocking.
Of course improvements to cabling, rack and isolation, and power filtering all contribute to the final result as well. But I think the new hifi sound you mention is a result of a combination of the main points above.