Powered Amp Speakers

Why does this say you should connect it to powered speakers? I thought amplifiers were only for passive speakers.

I will be switching a lot between powered speakers and headphones. Currently I use Audioendine D1 DAC/AMP combo which detects when I plug in headphones and cuts the audio to the speakers. If I'll go with this DAC, combined with an O2 headphones amp, how do I switch from speakers to headphones so only one of them will be heard?

Are the RCA outputs simply a line-level pass thru or is it a variable output so that this can also be used as a preamp to control the volume into a power amp or amplified speakers?

I see this has a line-out port. Is that impacted by the volume control ? Does it need to be sent to a pre-amplifier or can it be sent to a power amp directly for speaker use ? Thanks

Speaker recommendations for low-power amp (Micca Origain AD250)

Class D amp to power Harmon Kardon iMac speakers

If this has a 200hz roll off on the sub out , where would i set the crossover on my sub ? My set up at the moment is a - Topping E30 Dac / Nobsound NS-10P Tube Preamp which feeds into a Wharfedale SW150 Active sub through RCA input/output back to a SMSL SA 98e power amp. Then to ELAC Debut 5.2 speakers. Speakers go down to 46hz so i just set my sub at crossover at 45hz and the rest of the frequency gets sent to the main amp. Question is if i were to buy this AO200 or DA-9 with the 200hz roll off and a sub out port what frequency and what would be the best wat to connect up my subwoofer? Or would the sub out be useless for me. Any help would be appreciated.