If you wire a 1 ohm DVC sub in parallel, the measured load to an amp will be .5 ohms. There are amps that run in that range but it puts a strain on them. The lower resistance (lower ohm load) will allow the amp to move more power through the sub, but also generates exponentially more heat within the amp.
I've seen factory diagrams of old Rockford Fosgate Punch Power series amps running 16 4-ohm subs all in parallel. Crazy, but they are designed to do it.
A rule of thumb is that every time you cut the load in half, you double the amp's output (although distortion increases). Most amps are stable at 4 ohms. Some two. Rarer are the amps that are 1ohm or lower.
One more note - if you "bridge" an amp's outputs (combining two channels into one by using a positive from one set of terminals and negative from the other set), you will cut the ohm load in half as well (again doubling the power). So two 4-ohm voice coils speakers wired in parallel and bridged on an amp, the measured load the amp sees will be 1 ohm. You cannot bridge a mono amp because there aren't two channels to combine.
All that being said, much of the heat also has to do with how hard you drive the amp/sub.
Also, when the load is measured, it is when the speaker is at rest. When the speaker is playing, the measures load continuously varies in conjunction with the music it is playing.