[EE] DAC-to-headphone-level output amp

I'm designing a one-off device for myself that will generate audio from 
a microcontroller and I want to send that audio to a small BLE audio 
transmitter (something I picked up on Amazon).
The BLE transmitter has a 3.5mm headphone plug and they specify it's for 
"Aux" level devices, but works very well when plugged into my PC or 
cellphone, so I take that to mean headphone level.
FWIW, the audio signal is mono, but I want to drive both left and right 
channels.

Researching online gets me a whole lot of different answers -- 
everything from line level being 10k ohms input impedance and 1V or 2V 
to 3V and headphone level is 8 to 600 ohms, with voltage level from 0.3 
to 0.8V (though I would've expected headphone level to be higher since 
it's amplified).

Can anyone shed some light on this please. I'm unsure what I'd be 
designing for... should I just do an op-amp with a very low output 
impedance and adjustable volume until it sounds decent? Or is there a 
spec I should follow?
Alternately, I'd be happy if there's some simple monolithic chip that 
would do this for me (take a 0V-3.3V audio signal from a uC DAC and 
convert it to headphone level audio).

Cheers,
-Neil.