Congratulations on the 990 purchase! You made a great choice. They won't sound like the 325i's, as the 325i's have a rather magical mid range. The 990's are better all around phone, though, and have fun doing many types of music. I feel they are a much more versatile headphone in many ways. They will give you deep solid bass, much more so than the 325i's. The treble region will be much tamer than the 325i's, while still retaining ample detail. The comfort level will be something Grado owner only dream about, as witnessed by how many of them mod their phones to use the Beyer pads, headband, etc.
Happy listening!
- augustwest