The RE252 does have more bass. The bottom and top end of the RE252 does exceed that of the RE262, both in response and presence. When you use both back to back, this is blatant. Now the RE262 is more dynamic and has an effortless presentation of bass. It's clean, powerful, and could be a lot more pronounced if only notes carried more heft. The high dynamic range of the RE262 is deceiving though, and depending on what you're playing, one may consider the RE262 to be stronger than the RE252. It will depend on the source information you're using. The RE262 has better dynamics than the RE252. The RE252 has better texture and note thickness than the RE262. These benefits suit different things. As well, many people's idea of "bass" is high bass, information 80Hz and higher. Information below 80Hz is actually quite low sonically, lower than you think. A lot of the bass notes one hears in music are often higher in frequency than they would guess if they had to put a number on the note.
The frequency response I do is me using a pink noise track and adjusting what one could closely call an infinitely adjustable EQ, although I typically only adjust 10 main frequencies: 31Hz, 63Hz,125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz, 16kHz. The pink noise track provides a relatively unbiased look at the sound, and all one does is attempt to match output intensity across the frequency spectrum so 100Hz is as loud as 1kHz and as loud as 10kHz. It's just constant static, and you just move the EQ up or down till no one frequency overshadows any other. Low frequencies specifically can be hard to hear. Not all earphones present a well bodied bass note that is easy to hear, plus our ears and mind really are not geared to listen to bass with high accuracy. We are mostly geared for midrange (vocal) listening as a natural byproduct of human communication. There aren't many natural things that require subsonic accuracy, nor are we geared to hear ultrasonic frequencies, although some animals are geared to due to their "food" communicating at those frequencies.
Like xtasi was pointing to, frequency response is not the only part of sound. The presence of the note still totally depends upon how the note is presented, basically things like texturing, dynamics, decay, impulse. An example is the top end of the RE252 and RE262. In response alone, the RE262 is louder, but from practical listening, you would never tell me that. The ER252 is far more aggressive and edgy up top. There's more energy and air to the high end notes, and compared to the very light and sweet treble of the RE262, the RE252 sounds significantly more pronounced. Difference in presentation is different in perception. The actual sensitivity (frequency response) is only one part of the note development.