For control of Hifi devices separate from Alexa’s speaker, you will need something that can integrate with Alexa but send Infared signals to your hifi, like your remotes. I use Harmony Hub for this purpose, it works well as a bridge between Alexa and many other devices. I can say Alexa, play CDs or Turn on TV the sound all goes through my stereo, but when I ask Alexa for the weather she answers through the Echo speaker.
I hope that helps.