Very interesting query!
Copyeditor with 10 years under his belt, for what it's worth. Still learning new odds and ends of the English language daily.
As to this: Since the rule of a parentheses enclosed word is that the sentence can read perfectly fine without it--it merely adds value or some clarification, or what-have-you.
Thus, you would use the indefinite article that matches the noun--not the modifier in the parens. So, "I am having a (acoustic) piano tuned, yet again."
Think of it this way--"I am having a piano tuned, yet again." or, "I am having a piano (acoustic) tuned, yet again."
Whenever you run across an issue with those inserted sections of a sentence, be it between em dashes or brackets or the aforementioned parens, the best bet is to simply read the sentence without that section, and work the grammar that way. In the end, the sentence has to make sense without the dropped-in section, or it's inherently flawed to begin with--and your troubles have just begun!
Excuse the loquacity. Hope this helps.