Different amp topographies react differently with different loads. Some amps wouldn't have a problem with those speakers in parallel. Other amps are even more finicky about capacitive reactance or other complex impedance anomalies. But some amps will drive just about anything. Sorta like car engines... a huge diesel-truck engine, a hi-rpm sports car engine, and an average passenger car engine are all designed for doing certain things best... providing brute force (pulling) power, quick (racing) torque, or gas mileage. So you're right... it's all basic physics.

I agree with This Guy that it's possible the impedance could drop to 1 ohm or less at certain frequencies... the amp might be able to handle that only at very low volumes. I'd play the system with just one speaker per channel. Then connect them in series... it's not something that should give you any better upside, but it's worth trying... you might like the sound better (depends on the music and loudness you prefer).

Using power resistors has some downsides, most of which are dependent on the amp circuitry and how loud you plan to run it. It's not an elegant solution for sure, and could even blow out certain components under high stress unless you know some simple Ohms-law stuff.