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What kind of speakers do you have (brand and model number)?

Some lesser speaker brands skew their specs to the point that they are quite inaccurate. The only way you will be able to get 20hz from a speaker is for you to have a large subwoofer built into the speaker. If this isn't the case, then I'll suggest not going by the manufacturer's specs.

Keep in mind that you are not going to here any frequency at 20HZ. If you have a sub woofer connected to the receiver it will send all the low frequency to that speaker and cut off any low frequency going to the the other speakers. Normal it is cut off at the 80HZ and below going to the sub woofer and above that to the main speakers. I would not pay much attention to those specs that some manufacturer put on there products specially if they are not the high end products. Home theater magazine does review and test each speakers they can put there hands on, this would be a more accurate rating that the manufacturer. Hope this will help you out.

No there isnt a problem in that, as long as you adjust the cross over frequencies appropriately you will still get good sound. even though the sub cant go as low as 20hz which you probably will never have anything playing that low, it will still be a dedicated subwoofer, and not a full range woofer that will probably have its sound muffled trying to hit 20hz along with hitting all the higher notes. most manufactures claim to produce xx frequencies but in reality they only produce that sound for a split second and cant sustain it. if you are using a sub i wouldnt bother having your towers hit anything lower than 50hz or even 80hz.