On my dsp1124p, I use the TSR (1/4 stereo headphone plug) with a mono 1/4 adapter to RCA. I don't have DIN plugs on my 1124p, are you sure that's what you got ?
Also, you can EQ two subs with one dsp1124, it has two channels built in. Use the left channel for one sub, and the right channel for the other.
You can buy the 1/4 to RCA adapter at radio shack and many other electronic retailers.
Post up a link to a photo of the unit back or manual and we'll try to help you get this conneted.