The 2500 have fuller, deeper bass and IMHO sound more balanced than the 325i.
That said, the high frequency extension and detail of the 325i is pretty tough to beat, more refined, extended and detailed than the 2500.
For a unique set of headphones in their own right, I thought the 325i had more going for them than the 2500--at least when I owned the two of them together.
Also, I strongly preferred the Proline 750 to the 2500, and quite possibly to the 325i.
The Grados really are a treble-dominant listen; with a decent source and amp they can sound downright silky in the high frequencies, but it's not a balanced sound; bass is fast and punchy, but it's mostly a "midbass mirage." The 325i simply cannot go as deep and pronounced in the bass as the 2500.
IMM, the Prolines were everything the Grado SR series aspired to be, but the Ultrasones, I thought anyway, "beat the Grados at their own game," the only exception being the 325i's superlative high frequency delivery.