Sound quality is of course the most important quality, yet remains quite subjective. You can perform measurements and show frequency curves, but that says little about the listening experience. However, we have listened with enough speakers and headphones under identical conditions to the same song selection, that we are able to evaluate the sound characteristics compared to other similar products.
The TDK ST-700 sounds a bit muffled, with the middle and low tones sounding better than the highs. The sound is pleasant, and irregularities (or compression mishaps) disappear softly into the background with solid bass and the smooth sound that also characterise the aforementioned Solo HDs.
The ST-700 reproduces a cohesive sound signature, in which you can distinguish the various instruments. Mainstream genres such as pop and rock sound the best, but with classical music we miss the high frequencies a bit, and the more dynamic sections sound a bit flat. There is a lack of definition in the highs and mid-highs, which causes some songs to come over very soft.
This doesn't mean that the ST-700 are poor headphones. To the contrary, it's several levels above cheaper models and are about as good as the popular brands that spend as much on marketing as they do on sound quality.