Hi-Track, on Cabildo by the in-bound Jose Hernandez station of the subte D, sells audiophile gear, much of which is the same price you'll pay for much lesser stuff at Frávega. The folks are very knowledgeable as well.
I bought a Monitor Audio dock and speakers from them that's really excellent and wasn't very expensive. They also have a super-cool new LG box that docks an iPod and also handles CD, DVD, radio, etc. It even has iPod-style controls with a touch-sensitive disc for all functions. And unlike most of the iPod dock-speaker sets I've heard, it actually SOUNDS good, although not as good as the Monitor Audio, at least to my ears.