Impdence Question

It completely depends on the amplifier involved. Some amps do well with low impedance loads, others don't.

Among the amps that don't do well with low impedance loads the results can vary from distortion or the protection circuit engaging all the way to damaging the amp by shorting the output devices.

Check your amp's owners manual.

The only practical way to increase driver impedance is to either wire two drivers in series or use a transformer. (Transformers are normally used on tube amps for impedance matching but they can be used on solid state amps, too.)

>> there is concern about their low impedance damaging the
>> amplifier. Driver impedance ranges from 2.8 ohms to 3.4
>> ohms. Will this do harm?
it depends on the model of the amplifier. To get a hint of its performance look @ the amp's o/p specs. Does the output power double each time the load resistance halves? if it does,it would indicate an amp with a fairly robust power supply & the chances that such an amp could handle a lower speaker impedance are reasonably high.
if the amp power does not double the amp could still drive this lower speaker impedance but the amp will run out of juice. This'll manifest itself as lacklustre reproduced sound. If you crank up the volume, the amp could clip resulting in a distorted sound.

>> And if several speakers are used in a 5.1 surround
>> setup, does this increase the load and the chance of
>> damage?
should I read this to mean that you are going to connect 5 speakers to 1 pair of binding posts????
If yes, I'd say that's totally assinine!!
If no, then you must be using a 5 channel amp that has 1 pair of binding posts for each speaker + an output for the sub. In that case, you have 5 independent amplifiers each driving just 1 speaker so the question of overload does not occur if you keep the volume control within the limits of these amps per the amp spec sheet. Yes, there are all getting power from 1 transformer so each is "connected" to all the other amps & all 5 amps are not truely independent but this is a 2nd order effect for this discussion

>> If the low impedance is indeed risky, is their any
>> means of raising it perhaps by placing something in the
>> chain
you can create a Zobel network which'll "fool" the amp that it's seeing a (more) constant load vs. freq but I would advise against attaching such a network if you are not a serious DIYer or if you do not know much about speaker design or if the speaker was never meant to be used w/ a Zobel network. You will surely alter the sound (more often for the worse but you might get lucky & change it for the better! who knows...). Of course contact the speaker manuf for any advice they can offer re. Zobel networks for your speaker OR see if there is any discussion on your speaker model in some audio forum.
The other solutions are to either get a better amp to handle the lower speaker impedance OR get another speaker that is an easier load to your amplifier.
FWIW.

"The only practical way to increase driver impedance is to either wire two drivers in series or use a transformer."

Do not connect speakers in series because it will lower damping by amplifier (speakers isolate each other from the amp) - poor bass definition.

...Do not connect speakers in series...

Point taken, but this is a "pick your poison" issue. I'd take possibly making my bass quality a bit worse off as opposed to burning out an amp.

You will recall that our original poster stated that "only a certain high-end car speakers fit the design criteria." It sounds like he has looked for other drivers and not found any. In situations like that, one is often left with choosing the lesser of two evils.