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


I recently acquired an old Grundig Majestic console radio/turntable. It's an all tube mono amp with - as best I can tell so far - three speakers. With its FM section it's a surprisingly nice sounding amp, in a sort of lush tubey sort of way. It does have an unused input, but of course it's also mono. 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.

Regards,
grimace
09-22-15: Zd542
"If the OP uses software to convert stereo to mono, there is no hook up question."

Yes, there is a hook up question. Just because the signal is converted from stereo to mono "at the source", how would the OP get that from his DAC to the Grundig? You never addressed that in your original comment.
At the risk of sounding wishy-washy, Almarg is right - it's better to work into a load for any output - otherwise, you're basically shorting the left and right channels to each other.
"Yes, there is a hook up question. Just because the signal is converted from stereo to mono "at the source", how would the OP get that from his DAC to the Grundig? You never addressed that in your original comment.
Mofimadness (Threads | Answers | "

"It does have an unused input, but of course it's also mono.his Thread)"

Where am I going wrong? Why can't the OP just use the input?