If you don't mind a layman's opinion, I think if you can get hold of the response graph of both speakers, it may give you better opinion.

If your front speakers' response started to taper off at around 60Hz, you may need subwoofer's assistance from that level on wards.

I think unless its of higher end sub, otherwise most sub started to taper off at around 35hz as well. So if you can set from 40hz crossover, your sub's usefulness seems to be quite redundant in this case.

If you can't really get hold of the audio response graph of both speakers, maybe only your ear will tell you your own answer.

Please any audiophile here can rebuke me on this, just my own opinion.