This is a hard load for an amplifier

The minimum of the impedance curve is below 3.2 Ω. Nominal ratings are generous (a "4-ohm" driver commonly dips to 3.2 Ω, and an "8-ohm" one to 6.4 Ω) but a dip below this is worth knowing about before you commit, especially with multiple drivers wired in parallel.

Why the minimum matters more than the nominal figure

An amplifier's difficulty is set by the worst case, not the average. Current demand rises as impedance falls, and the dip in a multi-driver system usually sits in the upper bass where there is plenty of musical energy. Protection circuits trip, class-D output stages current-limit, and amplifiers run hot.

What to check

• Wiring. Two 8-ohm drivers in parallel present 4 Ω nominal and dip below it; in series they present 16 Ω and are trivially easy to drive. Baffle models both: try the series option and watch the impedance curve.
• The amplifier's actual rating. "4-ohm stable" is a real specification. Many home-audio receivers are not, whatever their power figures suggest.
• Cable. A long run of thin cable adds series resistance, which raises Qes and alters the alignment as well as wasting power. This is one of the few places where cable gauge genuinely measures.

What this is not

It is not a fault in the design. A low impedance means the driver converts current into force efficiently, which is usually a sign of a strong motor. It simply has to be matched to an amplifier that can supply the current.