Which is more efficient and requires less power?

Regardless of the speaker nominal impedance, impedance curves, or phase angles, what you will need at your listening position of 8 feet for an SPL level of 89 db is 90 watts for the 94 db/W-M speakers and 285 watts for the 89 db/W-M speakers. This is assuming 15 db peak headroom which is reasonable for classical and jazz.

But which speaker requires more power? Can't tell. If the 89 db speaker stays flat, then all it needs is 285 WPC. But if the 94 dB speaker drops down to 1-ohm at some frequencies, then it may need 360 WPC or more to prevent clipping while delivering the same SPL as the 89 db speaker.

Nominal ratings are a guide, but minimum and maximum power requirements are spec'd out by the manufacturer for these reasons.

Tvad, you have to take into account that as the distance from the speaker increases the SPL decreases. That one watt of power is at a distance of one meter, not the listening position (about 3 meters). Doubling the distance decreases SPL by 3db. Also, the headroom required to reproduce the type of music listened to is a factor. The SPL db at a given distance is (desired SPL - sensitivity) + 20 LOG(Distance to speaker/1 watt) + Headroom dB. The power required is 10^(SPL dB/10).

Your numbers are correct - but only at one meter with a 3 dB headroom. At 3 meters with 15 db headroom, it's 90/285.

The position of the volume pot is indicative of gain, not power. Power consumption is only done by the speaker as it draws current from the source amplifier which has a constant voltage, power supply permitting. The draw is determined by the impedance at any given frequency and the amp has to be rated for the power the speaker calls for.