To switch between headphones and speakers as an active output on the FiiO K7, you need to adjust the output mode. For headphones, use the «PO» mode, which allows adjustable volume from the front panel headphone outputs. For speakers, switch to «LO» or «PRE» mode, which uses the rear panel line-out. The «LO» mode provides a fixed volume, while «PRE» mode allows adjustable volume. Use the toggle switch on the device to select the desired output mode.

You can use the line out (PRE/LO) behind the device and connect active speakers, it's made exactly for that.