Objective 2 gain options + a few other questions about setup

Asking for help with Objective 2 headphone amp options. Would mostly be using Sennheiser 6xx and AKG 7xx Massdrop headphones. Source is a Harman Kardon HK3490 (with a built-in DAC) that I'd connect via RCA's. I live in the U.S. so assuming I'd go with the 110V standard version (1.0 x 3.3), is that correct?
I'm also assuming (perhaps wrongly) that digital content (Spotify and iTunes) would be processed by the DAC in my receiver.
Another question is whether this unit would improve the sound quality measurably. The receiver has Dolby Headphone. Per HK tech support (via another messageboard) the HK3490 has a" 270 ohm resistor in between the speaker outputs and headphone output. The power amplifier in this model is discrete circuitry and the preamp is op-amp based."

Aug 10, 2018
110V is a definite. 1-3.3x gain should be enough, unless your dac is underpowered, your headphones are ridiculously hard to drive or you like to conduct rigorous tests on your eardrums. DAC in use depends on method of connection. Measurable improvement will be for yourself to hear out :p
Aug 9, 2018
If you are playing Spotify and iTunes on your receiver now, it will work. If not you’ll need to figure out how to get those streams to your AVR (network, Bluetooth, coax, optical)
The o2 can connect from the rca tape or video out on the receiver. I did this for kicks with my 1610, but it’s not where I listen to headphones.
I haven’t heard the 3490. If you aren’t happy with the headphone amp on it, an external amp is worth trying if this is your primary listening area . The DAC is probably fine.