Impedance, yes. More so with an all tube amp, but even with transistor amps it's a good idea to keep the impedance above the amp's minimum safe load.
If there is any question about a speakers impedance, this is what I suggested in a recent similar thread; it's so inexpensive that IMO there is really no reason
not to follow it:
I suggest that anybody doing any wiring work, particularly gigging players that may need to "wing it" on the fly in an emergency, get a multimeter to avoid catastrophes. Wall voltage, speaker impedance, battery life, cable continuity, and many other things can be reliably checked out in short order.
Particularly with how inexpensive they are; here's one for $3.50.
http://www.harborfreight.com/7-function-digital-multimeter-90899.html
You probably spend more than that on gas getting to a gig. 
Also, the battery test detailed here;
http://sunn.ampage.org/sdp/index.php/topic,5585.0.html
is a good way to make sure everything is copacetic before you apply big power to your new wiring.
Also, since you used the term “loose speaker” I’ll mention that a loose speaker; as in not in an enclosure of some sort, will cancel itself out. There is as much sound pressure coming from the
back of most drivers as the front, but… 180 degrees out of phase with the sound coming from the front.
If you don’t isolate or somehow otherwise control the back pressure, the two can largely cancel themselves out.
