So set the multimeter to Volts ac. Connect black lead to the black post, and red lead to the red post on the speaker terminals on back of the speaker as shown below. You need not disconnect anything. You can probably put it right into the middle of the terminal going by this picture. You'll read a voltage. Likely less than 10 volts at a comfortable listening level which you will have already established with one of the DACs. If the meter isn't auto ranging set it to the highest voltage setting and reduce it if the voltage is low enough.
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So looking at the DACs you can use RCA with the Grace and XLR with the Octo. You could then switch between them at the Lyngdorf amp by switching inputs. Make sure you don't have to turn off the amp before swapping from RCA to XLR input. Doing that with the amp on might result in a pop or other sound you don't want going to the speaker. To be safe switch the amp off swap from one input to another and turn it back on.
One other possible issue is it looks like both DACs control volume to the nearest db. Worst case you will end up with signal different by .5 db, but likely you'll get closer than that. Though whether you'll get within .1 db is not certain. Hopefully you'll at least get within .2 db for your testing. You could do this by burning a CD-R if you have that ability. Trim the file level in software, save and burn the CD. But if levels are close enough you'll not need to do that.