Headphone amps (and amps in general) don't implement PEQ because PEQ is a digital process, so its more appropriate to implement in the DAC. The
RME ADI-2 DAC FS implements PEQ in the DAC; it does not have any analog input. The
RME ADI Pro FS R does have Analog inputs and can apply PEQ to them, but at an additional $700 to pay for the A/D converter.
Your position that all headphone amps are useless unless they contain A/D-D/A conversion (or at least D/A) is little like saying all cars are useless because they can't float on water. That's what boats are for. The RME's are
amphibious vehicles, and I
love them for it, but I can still appreciate a nice car or nice boat for being good at what they are designed to do.
PEQ is expensive to implement in silicon right now but in a few years it probably will be built into all the DACs. The
Qudelix-5K is showing the way (it's only ~$100 vs. $1200 for the RME, but you have to supply the computer (phone) to run the user interface
). Computer
software like Roon and SoundSource are making PEQ more accessible to use (it's been
built into the Mac OS for like, forever as a plug-in but the UI is
painful ).
I can appreciate that you perceive having a separate DAC and amp as a waste of space, but I can't agree that an amp is useless because it's
just and amp.