There seems to be some confusion regarding the meaning of "sweet spot". Here are a couple that I like:
The "sweet spot" is the optimal price-to-performance ratio where investing more money yields significantly diminishing returns.
or
In audiophile terms regarding budget and diminishing returns, the
"sweet spot" refers to a price range where you get the maximum perceived improvement in sound quality per dollar spent, before the cost of further upgrades begins to far exceed the marginal sonic benefits.
In other words, "sweet spot" does not mean the best possible sound that can be attained. It seems that most, but not all of us understand the term.