Dayne,
2.5 ohms is on the limit I have chosen for these devices. It would work fine, but for a bass amp alone I'd be happier if it were closer to three ohms. Remember, the XLS is a very high output speaker driver, with prodigious bass SPL.
At these very low impedances amps work very hard, supplying a lot of current with a large net voltage drop across the device. This often threatens to enter the unsafe zone of operation and destroy the device.
My comments are not inconsistent; merely cryptic. Consider; we have 49 volt rails, and a 2.5R load. Let's put 8 amps into the load, four amps supplied from each of the two output devices in parallel, alternately from both rails. This puts, in non-reactive terms, 20V across the load. With 49 volts on the rail, say a couple of volts of sag due to heavy current draw, (47-20) volts across the output devices. With 27 volts across the device and 4 amps flowing through it, we are sailing close to maximum SOAR allowance, which at this voltage, consulting the curves, is 6A. If rail voltage should be a couple of volts higher, you can knock half an amp off this rating. Secondary breakdown is caused by exceeding SOAR ratings, if only for a few milliseconds, and is the Archilles heel of bipolars, which otherwise are fine devices. A safety margin of around 50% on SOAR is something I like to gun for where output stages do not have protection; it's excellent insurance.
So, the basic answer is this: 2.5R is a safe minimum with these woofers. But really crank up the volume and it might still be possible with an unthrottled, unprotected output stage to exceed SOAR ratings and blow up the output stage.
Unfortunately I'm the first to admit I'm fallible and make mistakes all the time. Mostly I get it right, but now and again I do confuse people! I apologize if this threw you.....
Cheers,
Hugh
