Speaker impedance patterns - how to read them?


I know this has probably been discussed ad nauseam, but here's my question. I'm grateful for any sensible opinion.

Premise.
A speaker system may have a complex impedance pattern that can vary from 3-ohms to 30-ohms depending on frequency. Some speaker systems with similar impedance patterns may be stated as being 8-ohm loads because the speaker is 8 ohm in an important part of its frequency response, but only there.
A tube amp, as a voltage amplifier, likes a regular impedance pattern. Almost no speaker system has a constant, regular impedance pattern. I am aware that solid state, powerful amps are built to compensate for modern speakers' complex impedance patterns, but there are tube amps around and people who love them. It's difficult to understand what average load a given speaker system will present to the amp.

Question.
When looking for suitable speakers for a tube amp - mine is a PrimaLuna EVO 100, 40w x 2 from EL34s - what must be looked at? How to best interpret the impedance pattern, whenever available? Assuming that one listens at normal levels in a medium-sized room, what are the criteria for matching speaker and amp?

Thanks for your views. M.
martinguitars
@martinguitars, 
You asked a very reasonable question and have received some excellent and thoughtful replies (@audiokineisis as one fine example). Don't become discouraged and just ignore the immature and non useful comments.  I have no doubt that you'll find an excellent speaker match for your Primaluna amplifiers. 
Charles 
One other thing needs to be stated: don't match/buy a speaker for a certain amplifier based solely on the published specs that come from the manufacturer.  This applies to nominal impedance (a real moving target), sensitivity, whatever. Notice, too, that sensitivity can be expressed in different ways.  When magazines with the technical wherewithal measure speakers, the results are very frequently different, and when they are different they are almost always less favorable to the speaker (i.e. it will be harder to drive for one or other reason).  
Learning more about technical specs is always helpful, but since what matters is they sound good to your ears. I'd put emphasis on simply trying different speakers and not get caught up in tech talk. Graphs won't guarantee them pleasing your ears. That amp is small enough to take to a dealer for  demo.

I've used a PL Prlogue, which is the forerunner to your EVO 100. Most of the popular brands are 87-89 db, and will work fine-for "normal listening."  As the manual suggests, try both 4/8 ohm taps.

I used a sealed cabinet, 89 db speaker with good results. 

 

Hi all,

thanks very much for the several interesting and useful replies. And for the kind welcome, I am new to this forum.
I've bookmarked a few links and will read the pages carefully; on the other hand, I am well aware of what some, like tablejockey, have said - it's not a good idea to rely entirely on specs, and that's what I am not going to do. My starting point with this thread was to take advantage of the solid background of many to bypass, for once, the usual criterium of just listening and to try and find a bunch of speaker systems that match well my amp on an electrical basis. To do that, I needed to learn how to correctly interpret an impedance graph.

I think that with an amp like mine - which I am keeping because I like it, sonically and visually - it may be better to use a speaker system with not necessarily an extremely high sensitivity - which in my experience is often paired with low impedance, at least in critical points on the FR - but with a regular, highish impedance pattern. I too think that 87/88dB or so and a minimum 6ohms should be good companions for my tubes.

Earlier this afternoon I listened again to a pair of Naim n-Sats, which I used to have; they are not at all of anyone's taste, but have some nice qualities. They are a sealed box and the 'woofer' is about six inches. I am not saying that I like them more than anything else, but they surely would be a sensible choice. Unfortunately, they have an idiosyncratic voicing and can only satisfy certain room acoustics. But I got a better impression of them from a more aware listening.

Thanks to the careful reading of a number of papers I didn't know, I now have a clearer idea of what to look for; one year ago I would have based my choice on brand, looks, number of drivers... Now I know better and I can demo speaker systems with a clearer idea of what can be ruled out soon and what can be tried at home.

Thanks for now for the precious help,
greetings from rainy Italy.
Max
@timlub

We are clearly talking past each other and it's unnecessary so I am stopping now.