I don't recall the AKG K7XX NEEDING an amp--it does not require that much power. You probably can purchase any amp on the market for it, but the most commonly recommended combination is the Schiit Magni 2 and Modi 2. If you're not interested, consider the Fulla 2, featuring the same DAC as the Modi 2. You could also try the ODAC + OD combination from JDSLabs as well. I've heard some good reviews about the uDAC 3 as well, but I have never tried them and review sources are incredibly rare.