The 2 big differences are the placement in the circuit and if it is part of the amp, it is running off a much higher voltage... both of which affect the sound. A pedal eq or even a separate rack unit eq won’t sound exactly the same. The way Hetfield does it with the Mesa custom dual 5 band rack unit is the closest you can get to the real deal Mark heads if using a Triaxis 2:90 rig. Mesa took 2 5 band eqs from Mark heads and put than in a 2 space rack and upped the voltage. This unit does sound better than the Triaxis dynamic-voice pre-set eq. Not sure about the ones on eBay. Have to check them out.