can bass tones harm a subwoofer?

Im just curious but can bass tones harm a subwoofer?

Do you mean just playing a straight tone? No playing a certain tone won't hurt a sub. A sub can play any frequency without it being damaged. The only limitations are the enclosure it is in. You can harm a sub playing too low a frequency in a ported enclosure but this is not because the sub can't handle the frequencies. You also can't exceed the sub's thermal and/or mechanical limits.

ok. My box is tuned to 30 hz. So playing 45hz at full volume for 5 seconds at a time will be ok then?

depends on the power input. It will be fine as long as the thermal and/or mechanical limits are not exceeded