The S.M.S.L SP200 is a headphone amplifier with THX techology that offers silent backgrounds and both balanced and rca inputs and lots of power for almost any headphone and sells for about $289.00. Refurbished models can sell for just over $200 if you want to save more money on this.
The Status Audio CB-1 is an over the ear headphone that has a msrp of about $80.00 but it’s usually on sale for about $50.00.
The RME ADI-2 DAC is a well reviewed DAC/AMP that sells for about $1200.00
Sennheiser sells headphones for almost every application as well as pro audio tools and more.
The HDVD 800 Headphone Amplifier offers symmetrical signal processing from end to end with short signal paths and built in burr-brown dacs with XLR in and out as well as RCA. Expect to pay about $1000.00
Moon Audio is an online seller of headphones, amplifiers and other accessories like cables.
The headphone subreddit is a subreddit dedicated to all things headphones and related.
Hart audio cables offers high quality headphone cables for less than boutiqe brands.