Lucky,
Lot's of variables, and therefore many possible solutions depending on the cause of the problem.

If the noise measurement you took was accurate, a level of mid 70s (dB) should normally cause you no discomfort, and should not present a risk in the long or short term to your hearing. However there are lots of ways to measure sound, and I have no idea which method/weighting you used.

I am thinking that 'standing waves' may be a problem in your room, where certain frequencies resonate freely in the room you're in, making the piano sound boomy at that frequency. You can check this by getting someone to play the piano (including that annoying range), and moving your head from the normal playing position, to somewhere else (a metre or two to the left, a metre backwards etc...) and seeing if the sound changes with position. You could try moving the piano's position relative to the walls and other reflective surfaces. Or as has been suggested, attenuating the piano by dampening it with a blanket or other means.

Another possibility is you have 'increased sound tolerance' which is a pathology of the hearing system. Using earplugs while playing is good if this is what you have.
(BTW - has your hearing been tested?)

Re earplugs:
Given the Etymotic Research filters are available to you locally, I would go with those. They are a high quality product. Don't worry about Phonak.
My plugs have the Etymotic Research filters, but with locally made earmoulds.

If it is just an acoustic piano, the ER15s should sound OK, depending on how loud you play the piano. If you are in fact playing loudly, then ER15s would sound fine. If you are playing softly, then ER15s would probably attentuate too much and sound a little muffled.My best guess is that they will sound clear, but a little 'different', they'll take a little while to get used to (coz everything is softer), but you will prefer the comfort.
I think getting someone to take a mould for your plugs is safer than doing it yourself. Basically, you can't see what you are doing in your own ear canal, and you won't know how far to push (safely). And you can't allow for obstacles (like wax) if you can't see it.
Catch is, it will probably be more expensive through the audiologist. But then, they can check you are fitting them properly. (Most people I fit with earplugs usually put them in incorrectly and need a little help to get them started.)

I think tuning the piano will obviously make it sound better, but I don't think it will improve your loudness sensitivity.

Regards,

Eugene