Can i use an 30 hz speaker with 20hz

You can put any frequency you like (even DC ) into a speaker - and it will try to reproduce that frequency, but whether you actually hear anything is the point!

And do you mean a speaker with a 30Hz resonance - or a cabinet that has been tuned to 30Hz? That will be very different.

I presume a cabinet then.

If it has been designed for a (minimum) low of 30Hz, it could lose control of the cone below 30Hz and so damage the speaker. Better not to risk it.

that rather suggests a general purpose bass speaker rather than something monsterous in the lowest octaves

Ok, understand.

Then you will have to design your cabinet to use that speaker at 20Hz - not simple.

And looking at the response curve, the o/p drops from around 95dB @ 100Hz, to 75dB @ 20Hz. That means needing a lot more power at the lower frequencies, which will most probably kill the speaker. Best to stick with the recommended frequency range given.

if you expect that speaker to produce 20hz at any serious volume you are going to be disappointed

The specs on that site give no idea of xmax which is usually helpful in getting down low 

it also doesnt tell you if that 30hz is at -3db, -10db or even worse - Id take it with a pinch of salt were I you

for home use, if you put it in an a small sealed box (which would push the resonance up significantly) then added a huge amount of eq it would probably be pretty reasonable.

I actually like the P-Audio stuff. I have loads of 18´s, 12´s and 2" drivers in my system. They are very tough & reliable. I don´t think that I´ve actually blown any, which is quite amazing. But then I don´t use them outside the recommended frequency ranges! 

I´m sure that speaker will be fine for your parties - just don´t expect 20Hz from it!