If you have bookshelf speakers, try placing them upside down. Sometimes this can help by changing the acoustic centers and phase properties relative to your listing position (ears).
Sometimes it'll do nothing at all. A lot of speakers are designed to be best with the tweeter at ear level, but some pick other arbitrary points. You should be able to tell quickly. I find it tames the slight edginess of my Axiom M3Ti's.
If your tweeters aren't flush mounted, you can place peel-n'-stick felt around the tweeter, a few inches or so...helps reduce some diffraction. You'd be amazed how many speakers benefit from this.