I'm looking at buying a new keyboard to replace my antique upright piano, and I want to get the best headphones for the job. The two best keyboard options seem to be the Roland SD-700SX and the Yamaha CP-300. Any advice on headphones to match? With spending over $2000 on the keyboard, $300-500 seems appropriate for headphones. AKG K701? Sennheiser 650? Grado SR125? Do these headphones require/greatly benefit from a headphone-amp, when sourced from a keyboard? Any experience/advice/opinions extremely welcome.