Good question and one that I try to avoid.
None of the new designs are efficient by traditional measurement standards. The KIT41 & KIT61 come in around 85-86db/1W/1M averaged across their usable bandwidth. The KIT261 & LCR are around 87-88db/1W/1M measured in the same way.
I think that traditional efficiency ratings are a poor method of determining what type of amplifier you need for a given loudspeaker. For one, companies really stretch these numbers. They choose somewhere nice on the FR curve to pick their efficiency rating. I'd rather see the entire bandwidth averaged. The low frequency portion of a loudspeaker is what limits you. The baffle step usually occurs around 500HZ and under that you end up with an in-room 2-3db loss. You can either compensate for it electronically or in the acoustic design of the speaker/box system. Either way you end up throwing away efficiency at 1Khz where most efficiency measurements are stated.
None of this is useful in determining what kind of amp to use for a given speaker. It doesn't tell you anything about the impedance curve and it doesn't say anything about how the loudspeaker will perform with more than 1W of power. All of the XBL^2 drivers compress much less under power so they don't sound like an inefficient monitor. Trust me... I've got a pair of 106db efficient horns and I've listened to a lot of inefficient monitors. The differences are astounding.
Due to the lack of compression under power the efficiency at higher power levels looks much better than a traditional loudspeaker. That isn’t reflected in the 1W/1M measurement. Look at efficiency ratings of most loudspeakers under 5-10W of power and you will see that efficiency-ratings plummet. T/S parameters change up to 20% under power. Most of this is due to driver non-linearities where the BL of the driver is changing due to the VC leaving the gap. Some of it is due to heating of the VC increasing DCR of the voice coil. Some of it is due to suspension non-linearities as the cone moves. The typical 1W/1M measurement and T/S parameters tell you nothing about how the system reacts under load.
Rather than quoting numbers that don’t reflect anything meaningful, we have decided to publish detailed impedance curves and recommendations for minimum power. The impedance curve is determined by the total system design, crossover + cabinet/driver so it tells you a lot about how easy/difficult a load the loudspeaker is to drive. The second thing we do is recommend minimum power levels. Any of the kits will run to their maximum SPL capabilities with 40-50W of power. I use a LM4780 based chip amp that delivers about 40W to test all of them. It provides more than enough headroom.
As a side note, I took a KIT41 over to Dan Schmalle's of Bottlehead. None of the SET amplifiers had enough power to drive them to levels I'd consider acceptable. They need more than 8W of Single ended triode to be happy.
Tube amps will work wonderfully as long as you use larger powered tube amps that typically have 30W & over. I've tried a couple Push Pull EL-34 based amps and they work wonderfully.
Wow.... I'm full of wind! Look at that answer!
