The HD660s does look interesting, I should say that driving the headphone with a phone isn't a requirement per se, just a nice thing to have. I realize 90% of most headphones I'd have to get an amp. I just hate there isn't a good phone with a quality amp.
A heck of a lot of smartphones have a quality amp on the audio chip and a circuit that does not really hamstring that chip. What their audio chips do not have is
quantity of output power. At bet there's the Fiio M3 that uses a similar audio chip but since it can have a circuit and small battery dedicated to just being a player, it can squeeze more out of that chip, which is still overkill for a high sensitivity IEM unless you're using a high impedance IEM that requires more voltage.
The reality though is that smartphones are a mass market product and their primary concern is making them usable as smartphones, not as high output audio players, which necessitates everything to get bigger. High power output requires using an output stage with op-amps (or as in other stand alone amps, discrete components, or driver tubes), which then has to get the signal from a DAC, and that's assmuming it works like a new generation Wadia speaker amp where the DAC feeds the signal directly into the amplifier output stage, without an analogue line stage from the DAC chip as in conventional designs.
And as it is dedicated players not get more than 12hrs run time, and that's with a slower CPU running a streamlined Android OS. Even if they can squeeze in mic contacts and a sim slot on something like the Fiio X5III or X7, if they have to run other smartphone functionalities, people would just go on and on and on and on about either a four hour battery life or a reeeeeeeeeeeaally slow and unresponsive OS. And even then people will have to try to put something the size of the X5III into their skinny jeans, so the market will be limited to audiophiles who don't mind slow smartphones and wear loose pants.
Oh and about the sim slot and responsive OS...getting that done on a smartphone will cost a lot. Sim slot means NTC certifications will be necessary, which is why the Chinese brand smartphones that have circuits comparable to the Fiio X1 are only available in China and North Korea (are those people who keep getting arrested in NoKor also audiophiles? because they can just buy them in China instead of throwing the US a diplomatic curveball) and hope your network provider lets it run once you get the sim card in it. And then there's the development cost for that and their own Android version to run properly.
And yet, even with the 100mW you can get off something like a Fiio X1, that's at 16ohms. Power starts dropping the higher the impedance gets (or even below 16ohms). So in the end you're still not going to put in the same amount of power into the HD660S as a good, compact desktop amp, let alone one running a very low noise, low distortion Class A circuit.
You might as well get an IEM to use with a 10mW to 15mW output smartphone (which as far as quantity is concerned is already too much - it's more a matter of making sure the impedance isn't lower than 16ohms or even 32ohms, but not higher than 100ohms), and then use a DAC, headphone amp, and fullsize headphone at home. Even with the smartphone as a music server.
