yes pretty much, 4 ohm subs are rated as 4 being the nominal (lowest) load that the amp will see.
different frequencies given to speakers and amp will play different ohm loads but little under the ohm rating of the sub, and a 4 ohm. sub playing from 20hz. to 100hz. will see 4 6 8 ohm loads, meaning not every note is 4 ohms, even with test tones 80 hz. might be the time when the lowest resistance is seen,
this is why people are surprised when an amp dedicated to mid range sucks a ton of current and starts to warm up, it could be that at 275hz. the speakers lowest 4 ohm or a bit lower is seen while 125 hz. is at 6 ohms.
all this is much more complex but your amp will see 2ohms. and if you take one sub away it will see 4 ohms.
the difference is starting with a 2 ohm load you will get more power and 4 or 8 ohm you will get less.
http://www.rockfordfosgate.com/products/powercube_about.asp
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