IMHO: If you're using low synth / octavey stuff you should build a fEarB if you're going lower than low A or so, and a fEarful if you're not.
There are no bass cabs on the market that will do a good job of putting out even the first harmonic of low A or lower at any volume without risking being damaged. The fEarful is flat to the first harmonic of low G0 or so, roughly and no worse than -3db by the fundamental of E1.
The other issue is that loud fundamentals under 100hz that can come with synthy stuff push a lot of air through ports and most conventional bass cabs are extremely underported.
Anyway, check out the fEarful thread

Definitely the way to go for this.