You shouldn't confuse resistance with impedence ched, resistance is the simple part of the impedence but you also need to consider the complex reactance in this instance (saw your car earlier btw
). If you're doing the maths that is.
For maximum power transfer you need to match the input impedence of the speaker and the output impedence of the amplifier although this isn't relevant as I havn't come across a single cable manufacturer which impedence matches speaker cables. It's of little relevance for such low values anyway so I'm going off track...
Briefly in laymans terms, the output voltage of your amplifier is what determines the volume of your speaker. A higher voltage equates to a higher volume and this is the case for all speakers.
Speakers which are regarded as difficult loads do require more power but since the output voltage can't be dynamic (or else you'll hear rapid and uncontrollable volume changes which would be rubbish) we need to supply more current to make up this additional power. In general, you need twice as much current for each halving of impedence as has already been described so
you are best setting your amplifier to 6ohms which will improve current output capability.
Be aware that the number quoted by the manufacturer is only the
nominal impedence (ie. the highest value it can take and where it spends most of it's time) but that impedence varies with frequency so if you're playing low frequencies through a speaker the speaker impedence will be lower - sometimes dropping to 2ohms or less for short periods of time. This is why big speakers that play low frequencies need much bigger amplifiers which can provide the current into them at low impedences without giving up.
Also it's worth reminding people that high voltages aren't particularly difficult to reproduce. High currents require far more effort in design and components.
I hope I didn't bore too many people with what was hopefully a fairly non-techy explanation.