Downmixer for multi-device setup

I am interested in finding a pre-amp / downmixer that is able to receive multiple inputs and give one output. I do not need this device to be a DAC as I already have a decent DAC.

I have found a number of devices that look like they could potentially work, but I have some questions and I thought I would ask the audio experts for advice on this.

The DAC I have is an “ifi Zen DAC V2” which has 6.35mm, balanced, and RCA output. I do not want to decrease the quality of this device’s audio at all so I am inclined to believe it would be best to use the RCA output when connecting it to another device (assuming there may be quality loss otherwise). The other devices I would recieve audio from are not HiFi and I would probably connect them with 3.5mm to RCA adapter cables. The output would be most preferred as 6.35mm, but I can do 3.5mm as well.

These are some devices I found that could potentially work:
(some are RCA output, but I figure an adapter could fix that)

(Same device? Private label?)

My concern with these devices is that they may be cheap quality and have poor sound, but I also do not know what is takes to make a good downmixer. I know you should be picky with your DAC choice, but maybe not with a downmixer?

I am also not against looking into DIY solutions too.

Any advice would be appreciated! Thank you for taking the time to read this!

Edit: Apologies, I had this initially listed under “Speaker Amps”