What are the weak points of Pass amps and pre-amps ?

One obvious advantage of high efficiency and easy to drive (i.e., uniform impedance) speakers is the ability to drive them with less powerful amplification and having a greater choice of amplification for the money. I wish I had considered that years ago but instead now have myself in a position where I need rather large (and therefore expensive) amplifiers. The cost of high quality amplification per watt of power goes through the roof when you need a lot of watts, and you can forget tubes unless you want to spend a fortune on tube replacement.

It would have been nice to be able to use something like the First Watt amps, the lower wattage 30 wpc Pass Class A amps, or any number of under 50 wpc tubed or SS amps available.

I am wondering why all speakers are not high efficiency?  Why make medium or low efficiency speakers at all, what are their advantages?

Kickbacks from amp manufacturers perhaps.

High efficiency or perhaps more correctly high sensitivity unto itself is an admirable attribute. High sensitivity can play louder, tends to allow lower amplifier distortion, reduce power requirements and ergo reduce cost,size, weight and heat. High sensitivity also allows more noise into the signal. On the other hand it’s easer to create a steady lower impedance than higher one. A steady impedance is easier on amplifiers, reduces the potential for phase angle challenges, and frequency response linearity deviations. There are many considerations that go into speaker design, balancing them all sometimes requires sacrificing the ultimate capabilities of some attributes in order to better accommodate other considerations. How many commercially available high efficiency loudspeakers capable of time and phase coherence can you name? How many high efficiency speakers can demonstrate a good step response or square wave?

Never buy anything from a human that is trying to sell you something. They are way too dangerous. Chord? Are you kidding me? The difference between Pass and Curl designs is that they actually care what their amps sound like and they actually know how to listen to them on top of knowing what their customers want. Frankly, if it is art you are looking for buy a Boulder, best low distortion art out there . Oh, and if anybody thinks they have to spend that much money to get incredible sound they are out of their mind!! Besides, it is a lot more fun doing it on a budget.