Maybe it's a silly idea but, If I'm not mistaken, the 3510 uses it's speaker for polyphonic ringtones. I guess that it just won't give that much output for lower frequencies.

I just tried to send a ringtone to a 7650. It seems to get the message, but doesn't do anything with it. The ringtone has been tested and works fine on a 3310. I don't know where to start searching...