Not retro itself, but I got this Amoon AM-228 mixer to hook up all of my retro MIDI gear and computers. It looks like a nice little device and was only $24 with free shipping on eBay (from in the US too!).
It is one of these:
https://www.amazon.com/ammoon-AM-228-Ultra-co … V522KF9XC0BEFFZ
I like that it supports 4 stereo inputs and uses a power supply. I'm not convinced that passive mixers like the Rolls MX42 would work well enough for a variety of old devices and they cost more than twice as much. The separate right\left controls may be a little odd at first, but it may actually help in my situation because my "retro area" in my office has the left and right speakers at different distances, so I'll probably make use of this feature.
I'll probably make a post about it here whenever I get a chance to set it up. Mixers are often recommended on this site, but I haven't seen this particular model mentioned. It is by far the cheapest 4 channel stereo mixer with the most features I could find. First impressions are that it uses a very sold metal construction, nice smooth turning pots and sturdy feeling switches. Its obviously a budget "made in China" device (instructions came on a single printed sheet of paper for two langauges), but the device itself doesn't feel cheap at all.
Now for some blitting from the back buffer.