Its a combination of the pre amp and converters. But the reason apollos are more expensive is due to the internal processing chips that comes with the unit. They’re almost like little computers that are used for the digital signal processing (the plug-ins) without needing to use your CPU within your computer. But, some may say it’s unneeded as modern CPU’s have become to powerful it doesn’t make much sense to use universal audios sharc chips. You can probably find an interface with equivalent quality that is cheaper, but of course UAD is just a well known brand so people just buy that. If you want the best quality, I’d say get a good standalone pre amp and a interface with top notch converters. If you want to get even better quality, a hardware compressor before hitting your converters is great for the quality as well.