1. PC Sound card: You don't need a professional card, but Audigy 2 cards will work. The main goal is to bypass the integrated sound card of your motherboard, if that's what you usually use. Professional plug-in sound cards use more expensive components, which will result in better sound. They will also include more ports and will come studio-ready for recording purposes for professional musicians.
Some of them come in external rack-mountable form, allowing for more customization than a simple plug-in card. Users will experience less latency with a professional sound card, and will be able to handle multiple audio inputs and outputs at the same time.
The better the DAC, the better the sound. Cheap integrated DACs are found in CD players and speakers, but standalone DACs create a better sound that can be fed into any amplifier, headphone or otherwise.
The Music Hall dac25.2 is a good option for users wanting a tube-based DAC. However, these kinds of DACs can cost more. It make sense to pair a tube DAC with a headphone amp that also uses tubes. Tube amps will help deliver the typical valve sound associated with vacuum tubes. In order to get the most out of this kind of DAC, you should rip all of your music files in a lossless format. That will allow the DAC to perform its work. The sound is less clinical, "warmer" than what's usually associated to sound coming from a digital source. You'll spend between $200 to $500 on a good DAC.
The simpler circuits produce less harmonic distortion. Some examples of headphone amps using vacuum tubes are Woo Audio's headphone amp and the Bellari Class A Tube Headphone Amp (VHA540). Headphone amps start out pretty cheap but they can get expensive quickly. Expect to spend $300 to $500 for something decent.
You can get an external sound card for your Mac, but to start off with, a good basic setup simply involves a DAC and a headphone amp. That will allow you to get the most from your higher end headphones.
2. Headphone Amp: As with PC setups, the headphone amp is a core part of any audio enthusiasts' system. The good thing is that headphone amps are audio hardware that's compatible with either Mac or PC systems.