For efficient speakers, how much difference does an amp really make?

93db speakers are not that efficient. They will take some power to drive. I use Altec speakers that are around 99db and drive them with Mac MC40 monoblock tube amps. At normal listening levels (loud) I'm not using even a watt of power. If I crank them up till I start to hear some distortion (clipping) they are way to loud.
When I got into this hobby I was always chasing bigger amps, more power, had in-efficient speakers and thought that was the only answer. After a buddy showed me what a few watts on some Klipsch horn speakers could do I had to re-evaluate my thinking.
I also wonder on an amp that is designed for say 300watts, just how good is that first watt of power. Are the solid state devices really that linear at levels just a tiny fraction of their rated power? At 1 watt it would be running at 1/300 of it's rating and on softer passages less that 1/3000 of it's rating. Tubes seem pretty immune to that sort of thing, at least my Mac's are.

Billwojo

For high efficiency speakers, amps ar a big deal. IMO, an amps output "gain" is key. Or should I say, the combination of your pre and amp's output gain. If really high (especially with tubes), you will be more prone to hear hiss, noise, etc with high efficient speakers. 

The gain structure of the current model Adcom amps are all below 30 db, I believe.  The 565SE's gain is 27 db.  When I was checking out the input sensitivity of this amp, I noticed what appeared to be an error as the Adcom spec page lists it at 12.1 volts input for unity power.  I emailed Adcom and they responded within a few hours, saying the spec is actually 2.1 volts.  By way of comparison, the 555SE is 1.7 volts.

I’ve owned both the Ascend Acoustics Sierra 1 and Sierra 2 with the Raal Ribbon Tweeter. The brand is meticulously designed and tested. The components employed are anything but off the shelf, and it shows. When I put the Sierra 2 up against my Paradigm Monitor 7 V3(a tower), I was almost shocked.

Shocked at the mediocre sound I had been putting up with for about a decade. But the Monitor 7 was merely designed to be a slightly better option to the big box options back in 2003, and the industry was turned on its head since then. Still, it demonstrated to be that the Ascend Sierra 2 was a serious speaker.

Anyway, I’ve never heard the Sierra Tower, which is a different animal from David Fabrikant’s various bookshelf designs. Typically they were all easy loads, benign phase angles, and relatively inefficient. I haven’t owned my Sierras for about 3 years now, so haven’t kept up with what they’ve been up to.