"OK, eventually I will have a DSP, hopefully within a month. But without a DSP an EQ on subs is pretty much useless?"
Yes. Your chances that the peaks in the subwoofer response fall on 31Hz, 40Hz, 50Hz, 63Hz, 80Hz or 100Hz are pretty slim. Plus they are not likely to be 1/3-octave in width.
"If you have no DSP or parametric EQs on the board, I would take an EQ, assuming you have one. I certainly wouldn't buy one specifically for that purpose."
I wouldn't buy (just) an EQ either, other than a bargain (on a well designed one) on Ebay. I would, however, consider a DSP "loudspeaker management system" well worth the investment.
"Without a DSP at all I would certainly want the EQ to 'high pass filter' the subs, sort of, just by notching out everything too low."
This may be better than nothing but it doesnt even begin to approach what a real HPF accomplishes.
Depending on how you push the system, a properly set HPF can mean the difference between your subs surviving or not.