To amplify the HD500 sound without affecting its tonality I think your best options are:

- a pair of studio monitors with minimum 5" speakers - up to 8"would be best. Make sure you get active (self-powered) speakers that are Full Range Flat Response (FRFR). The latter ensures minimal tonal affect on the incoming signal. This setup would be best for home studio use.

- a small, portable PA system with minimum 8" speakers, again FRFR would be best and is generally achieved by keeping the PA system's  tone settings neutral and not using any of its onboard FX (e.g., Reverb). This setup is useful in a home/practive environment and also for gigs in small venues that may not have their own PA system.

- a Power Engine amplifier, or a keyboard amplifier. Looks like a guitar amp but is designed to provide close to FRFR amplification. This setup is best if you plane to mic an amp into a soundboard.

I would not run it through the computer.

Hope this helps.

i got the Tech 21 Power Engine 60. love it. light, powerful, and all it does is amplify the sound. very affordable. got a used one in mint condition from Guitar Center online and it was under $300. cheap enough to grab two if you wanted and have 2 paths to play out of. im only using one, but i can dream.

http://www.tech21nyc.com/products/powerengine/powerengine60.html