Sub connection question.....

I currently have an older HT receiver with sub out connection and no x over adjustments...set at 100hz...will this automatically send everything below this frequency to my sub(internal crossover disengaged)?...I would also like my main speakers to handle just 100hz and above? other words...I dont want to "double" up lower frequencies..or do I have to "hi-pass" my system to accomplish this...for the record...I have large, full range speakers...but would rather have my sub handle majority of I have it configured correctly?
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Usually a HT receiver like this allows speaker size selections in setup. The main speaker selection may have choices like small + sub, full range only, full range + sub, etc. If you choose small + sub, everything above 100Hz should be sent to main speakers and everything below should be sent to sub.

Of course different HT receiver will have different features. If you can post the brand and model number here, I can try to find out how its setup works for you.
FYI...this is for Stereo mode listening if that makes a is the x over in my AVR a high pass fixed filter...or a low pass? sub only has one speaker level/ high in connection?...isnt a loop required for true hi pass intergration?....
As Sid suggested, why not tell us the identity of the AVR and
the subwoofer? Otherwise, any comments are merely conjecture.