The Bottlehead Crack 1.1 OTL Headphone Amplifier Kit is on sale again until 5/12 for $199 (normally $299). The Speedball upgrade may also be added for $100 (normally $115). This sale has historically occurred about every 6 months, though I don't believe it ever gets cheaper than this.
This is considered one of the best amplifiers for high impedance headphones such as the Sennheiser 600 series (many think it's the best), however assembly is required so you need to be pretty comfortable with a soldering iron. There are a number of assembly videos on YouTube if you want to see if it's something you'd be willing to tackle (e.g. https://www.youtube.com/watch?v=2h39DWLgHmE