Singapore is not a headphone heaven? I guess you're partially wrong.
At Adelphi,
2nd level - Music by Design
They are the main Grado distributor in Singapore. They have almost all Grado model in stock, from SR60 to the RS-1. Good service and the store manager actually let you try out any grado model before buying. There are other shops there that stock some AKG and Sennheiser headphones.
Sim Lim Square,
3rd level - Hung Brother
they stock lots of headphone brands (AKG, Beyerdynamic, Sennheiser, Koss, Phillps, etys. Their price are a bit steep but negotiable. Not all headphones models are available, I was told by the salesman there that they depend on their supplier stock, and they could not gurantee the availability of each and every models. I bought my portapros there at $89, I can't find it at any other shops to compare price.
At
http://www.bluetin.com
Base in Singapore and without any storefront. Carries mainly Sony and Audio technica. Pleasant and prompt service.
The only brand i haven't seen available in Singapore is the Ultrasone.
Zia, if you have heard the portapros, try Sony D66sl Eggo. They are very portable, lightweight and have awesome sound.
Good Luck!