Dan addresses this in his interview with Head-Fi on the Expanse (search youtube head-fi expanse). He states that the Expanse is less U-shaped than the Stealth, which he says conforms to Harman more closely. He says that with the closed-back Stealth, the lower FR in the upper bass was intended to help the soundstaging, but the same constraint was unnecessary on the Expanse. He wished to achieve an "experientially flat" upper bass for the Expanse, because conforming to the Harman curve sounds too recessed.
Below are the frequency response curves to compare. The Expanse is tuned flatter in the upper bass than either the closed-back Stealth or closed-back Noire. It appears on the Head Fi FR curve to be about 2 db +/- of warmth from about 90-400 Hz.
EXPANSE VS STEALTH: Stealth is more U-shaped from <100Hz to >400 Hz.
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EXPANSE VS HARMAN:
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STEALTH VS HARMAN: Stealth has a bit of the 'Dan Clark bump' from about 60-160 Hz, but the Harman-conforming U above that range. I think the Harman curve is fitted just a little higher on this chart vs the Expanse chart above, perhaps letting the Stealth "U" look a little diminished; the RMS level (vertical axis) bottoms out at just under 300 Hz for the Expanse and 550 Hz for the Stealth, as suggested in the Head Fi curves above.
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NOIRE VS HARMAN: The Noire has a firm bump, followed by a U above 160 Hz. The U partly conforms to Harman, and dips a bit below in the ~200 Hz and 700-1100 Hz bands.
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