Uh, well Mad's E9 is better as an amp than the E11, and selling for a fantastic price at the moment, though it's more transportable than portable. The Little Dot i+ hybrid amp is also quite renowned for $120 new, and the Objective 2 amp which you can build yourself from JDS Labs for around $100 or pre-assembled for $144 plus shipping. FiiO also has the portable amp/DAC E17 that is good on it's own but it can be docked with the E9 for a desktop amp/DAC solution. The E17 is $140 on amazon.
That's my shortlist, they're mostly small desktop amps and not something that can sit next to an iPod in your pocket (except the E17), others probably have other suggestions. Not sure how many portable headphones require an amp better than the E11.