I understand that you may not need to match the speaker's wattage exactly, but I'm wondering if the power level is PURELY an issue of volume or if there are other things to take into consideration.
The power rating on the speakers is the level at which you run the risk of damaging them. It has no practical meaning for home audio and should be abandoned by the industry. It causes more confusion than information. I suspect it is lawyers that motivate inclusion of this specification.
Does a speaker sound better at low volumes with a higher wattage receiver?
No. Unused power capacity is simply unused power capacity.
Will any damage be caused to the speaker by being under powered by up to 50% or more?
No. Speakers are damaged by overpowering them not underpowering them.
I also have heard that it takes more wattage to do low bass tones.
That's correct. The low frequencies dissipate most of the power. Because you have a powered subwoofer, however, your receiver won't have to deal with the low bass. The subwoofer handles that with its own amplification.
I've been looking around at receivers and the only ones that can do ~150 watts @ 8 ohms - 20-20khz are quite expensive.
The one you have is more than enough. You won't gain anything important with a different receiver other than features that you might want.
I also see threads where people state that it takes quite a bit more wattage to give a listenable difference in volume so that has sort of dissuaded me from picking up any receiver that does 100 watts per channel for instance as opposed to the current 70.
Every doubling of the power adds 3 db of potential volume. The difference between 70 watts and 100 watts is trivial. Also it isn't likely that your receiver will ever dissipate more than about 20 watts per channel.
Can anyone give me some guidance about what I should be looking for given my use case?
Not without understanding what you think is bad about the system