When you change the way a speaker (or group of speakers) is wired (series vs parallel), then you change the ohm load that the amp "sees".
For example, a dual 4 ohm speaker can show an amp a 2 ohm load (parallel) or a 8 ohm load (series). Add another speaker and the same math applies.
Two 2 ohm subs in series is a 4 ohm load on the amp, and in parallel is a 1 ohm load.
Unless an amp has a regulated power supply, an ohm load on an amp is kinda like a throttle on an engine- the lower the ohm load, the more watts the amp is allowed to output. Ohms = resistance, so the more resistance, the less the amp will output. Usually, more output = more heat, so there is a tradeoff.
Hope I didn't confuse you too much.