The Onkyo A-9555 is a very fine integrated you could find used or refurbished in your price range and it has a good headphone output. There's a full review of the amp on the Stereophile site.
Another option for greater versatility than a dedicated headphone amp would be one of the many DAC's that include a headphone stage. Both the Beresford and Aune DAC's I'm using now have fine headphone output, as did the more expensive Benchmark I had before.
Another option for greater versatility than a dedicated headphone amp would be one of the many DAC's that include a headphone stage. Both the Beresford and Aune DAC's I'm using now have fine headphone output, as did the more expensive Benchmark I had before.