Ok.. I had to watch a video on the Spartan. It's much like the current tanks, where the coils come in a cylinder type device, and the coils are in the center. This seems to be a dual coil setup. The ohms option of the coils are 1.5 ohms and 2.0 ohms, and they offer a Mini/ego (18.5mm) version and a standard 22mm version.
I've noticed that you mentioned your kangertech EVO battery being several years old. The bad news is, all lithium batteries have a limited lifespan. Over time, the ability for it to hold a charge diminishes.. .that's just the nature of lithium ion batteries. Kangertech offered several versions for the battery. The first thing I'd check, is if the voltage can be varied. If there is a spinning dial on the bottom, try lowering the voltage If there is no dial on the bottom, try pushing the button three times. if the color of the light around the button changes, then depending on the color, that's the different voltages. if the color doesn't change, then the voltage is fixed, and it can't be changed.
For vaping, experience, it's normal to match the voltage to get the best flavor and vapor from your coils. IF you can't change the voltage in your battery, then you try to match the coils to your vaping preferences. The Spartan only comes with two options, so if you are getting a harsh taste, try using a 2.0 ohm coil and see if that works better. Usually, they mark the ohms on the side of the coil.
I hope this helps. Without knowing exactly what battery you have, I won't be able to give too much advice.