Describe the "new HiFi sound"?

Sounds like B.S. to me.

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. 
 

About "musical."

Language can be used in all kinds of ways, so I will remain open to uses of "musical."

The logical part of my brain cannot stop hearing the "beg the question" fallacy in the use of the word.

Wikipedia:

To "beg the question" (also called petitio principii) is to attempt to support a claim with a premise that itself restates or presupposes the claim. It is an attempt to prove a proposition while simultaneously taking the proposition for granted.

When the fallacy involves only a single variable, it is sometimes called a hysteron proteron (Greek for "later earlier"), a rhetorical device, as in the statement:

Opium induces sleep because it has a soporific quality.

[The speakers sound good because they have a musical quality.]

Reading this sentence, the only thing one can learn is a new word in a more classical style (soporific), for referring to a more common action (induces sleep), but it does not explain why it causes that effect.

Hey, keep your hysteron proterons to yourself. 😄

All kidding aside, this comes up every now and then but there does seem to be a trend like blisshifi alluded to in his post. Case in point: my new speakers are much more revealing, have more air, ambience and room/studio presence in the recording but there's no etch, shrillness, or harshness to speak of. The mids are enhanced with brass, winds, strings, chimes and percussion that don't ever shade the sound but seem to burnish it, imbuing them with body that project out into the room. And the bass is propulsive, tight and expansive with all the sound emanating out from the speakers to the point where they can face with minor toe it, allowing a very wide sweet spot. 

All of that is only possible with new driver technology that hasn't been tried before. Throw in a wonderful sounding all digital amp that "hears" deeply into a recording and you have the recipe for some truly engaging sound.

That would be my definition of some "new hifi sound" as I haven't heard it performed this way in all the years I've been in this hobby. 

All the best,
Nonoise

@kota1 Sorry to burst your bubble, but that statement is totally false.