Generally speaking, the KRC-3 will not add any warmth on its own. It will, however, sound more real than the 250P. I own the KRC-3, and it is capable of being part of a very good and satisfying system. It will be more accurate and just plain sound better than the 250P. In fact, it is, IMO, much better than the newer 280P. Compared to the KRC, the KRC-3 lacks a little refinement (not quite as smooth), and does not image/soundstage as well. But you cannot match it with bad components and expect to get good sound. I am happy with mine, and currently think I can improve the system by improving the front end, amp, and speakers before the KRC-3's limits are found.
Personally, I might upgrade the source first, and optimize the setup before