Is it possible to down-mix stereo outs to mono amp

Yes, can be simple as using 2 resistors, along with with more
advanced ways of doing that.

Google "stereo to mono mixer".

Actually a simple "Y" adapter will do.

Mofimadness, yeah, you're right - I was just overthinking it. No matter what, he can't hurt anything by trying that.

"Is it possible to take the two channels out of, say, a DAC, and blend them down into a mono input? I'd love to be able to play streaming music or computer files through it."

Your playback software should have a mono option. If not, try Clementine music player. I know that player has it.

I agree with Joey's first post, but not his second. And this is the first time I can ever recall not agreeing with Mofi.

If ZD's good suggestion proves to not be applicable, perhaps because one of your sources is not computer-based, a y-adapter will probably work, but IMO it is very poor practice that, depending on the output impedance and other characteristics of the DAC, could eventually lead to its premature failure. Especially if that output impedance is particularly low, and if the DAC's output circuit is not robustly designed in terms of short-circuit protection.

Consider, for example, the situation in which you are playing a recording in which high volume transients are produced by an instrument that is off to the side, which are therefore primarily in one channel, and occur at times when little or no sound is coming from the other channel. You will then have one channel of the DAC trying to produce 0 volts, and the other channel trying to produce a relatively large voltage, the result being that the two interconnected channels will fight each other. Resulting in turn in an abnormally large amount of current having to be supplied by one channel, as it tries to drive a load impedance corresponding to the perhaps very low output impedance of the other channel.

As I say, it will probably work ok in most cases, for a while at least, but at best it is poor practice, and at worst (depending on the specific DAC design) it may result in premature failure.

My recommendation: If ZD's suggestion doesn't apply, either implement a two-resistor DIY summing arrangement, as Joey suggested, or visit the B&H Photo Video website and select and order a simple line-level mixer.

Regards,
-- Al