Yeah the nu3k will be too weak for the 24.... I mean: it will do for the short-term, but long-term... you'll very likely want a beefier amp.
The smell is from the glue and plastic that holds the voicecoil together, you probably (almost) cooked it. Don't be suprised if the SI-18 dies far early that expected as a result of that incident.
Bass in the >40hz region causes a lot of heating in voicecoils.
Bass in the <40hz region causes the driver to bottom out.
(But only at high power and/or for sustained durations of several seconds to several hours, depending on conditions.)
>40hz is the mid-bass region.
It sounds you could benefit from replacing the SI-18 with 2 P460's hanging off the nu1k @ 4-ohm.
Pro-woofers can handle mid-bass all day because their impedance curve is optimized for that, and it also helps that pro-woofer are 10db per watt louder than high-excursion ht/car subs.
PA woofers are often 20ohms in the mid-bass, where the car woofer will be 4-ohms, give or take. Thus way more current.
Voltage doesn't cause heat, current does.
That's why high-power lines are extremely-high voltage and run at room-temp, instead of high-current and thus 1000 degrees white-hot knives.
DC is even worse than AC in this regard; that's why car amps need quad-0 cables. Because: 12V and high-current.
99% of the power dumped into the voicecoil gets converted into heat, rather than generating EM force and thus cone acceleration, which only exacerbates the burnt-slinky syndrome.
It is always better to run at high voltage and high ohms, than high-current and low-ohms.
Ideally all woofers would be 8-ohm and amps would swing 400volts.
But transistors that can swing 400volts are really expensive, and so, 2 or 4ohm is more common and amps that only put out 100-150v (or 2-5ish kWs) are made.
The laws of thermodynamics and Ohm's law are a bia, huh?
Luckily we can just double the cones and amps for netting 6db louder. But you can only double-up so many times until you run out of money/space/circuit breakers. The house always wins and there is no free lunch.
Even in the DIY space, 99% give up after about 16 18's and 8 stereo amps.