Good point about speakers.
It probably makes sense to skew the master mix slightly toward the kind of speakers one's target audience will most likely be using.
I heard Elvis was a stickler for this approach, at least in the early days (see photos from '56 below). He'd preview his acetates on an inexpensive battery-operated record player, and if he didn't think they sounded good there, he have the master mix adjusted accordingly. I understand the Stones did something similar, rushing down to their limo during sessions with a cassette to see whether or not the latest tweak still sounded good on a car system.
The holy grail, of course, is a master mix that sounds great on both low and high fidelity systems.
I always thought that if one could afford the cost of multiple mastering mixes, this would be a great strategy, ie shoot for a master that sounds decent on, say, laptop PC speakers, iPod/iPhone headphones, car stereos, AND high end audio systems.
Difficult, yes; but if old timers could do it with analog equipment, clearly not impossible.
THEE DIRTYBEATS - maximum vintage garage
http://theedirtybeats.bandcamp.com