Concert Experience for $5k?

I listened to the ProAc Response D18 at my dealer and both the studio 125s (used) and my current studio 148s. I would have kept the 125s if the did not get loose on some of the lower frequencies like the D18s. The 148s did not have this problem. I use my system for 2 channel AV also. 

I suggest you seek a speaker with 8 Ohms impedance and at least 92-93dB sensitivity, and preferably higher. Speakers with such characteristics have a lot of liveliness to them. They are not the "end all, be all," but they would be able to deliver much of the impact and vibrancy you seek. You may wish to reject any speaker with 8" bass driver(s) or under. 

You may be a candidate for the newer large Cerwin Vega speakers which were said to be surprisingly good for the money; 6Ohms and 95dB - perfect. There are a few reviews, I believe on the Net. Now, I'm not saying they will be better than most of the others mentioned, but imo when you are playing music at too loud levels you are introducing all manner of distortions no matter what speakers you are using. Given enough level a high end speaker which is incapable of such performance can distort with the best of them. So, if you are to rock such as to knock the fillings out of your teeth, then why spend audacious sums to do it?  
So, go for it, and FEEL the distortion!  In all seriousness, it's another way of being a listener and getting into the music.  :) 

I would strongly urge you to reflect upon the following paragraph in Stereophile’s measurements of your amplifier:

...allowing for some variation in the calculated output impedance with the level and load impedance used, we’re looking at source impedances of 5.5 ohms (8 ohm tap) and 2.8 ohms (4 ohm tap) in triode mode. While these are both high, they are exceeded by the ultralinear figures: 12 and 6 ohms, respectively! Fortunately, the impedances don’t change much across the audioband, but, as figs.1-3 reveal, there will be a large and audible change in frequency response depending on which loudspeaker is used with the Cary and which transformer tap and mode of operation are used.

The review is at www.stereophile.com/content/cary-cad-280sa-v12-power-amplifier-measurements.

While many of the suggestions made above can be expected to be good ones in most circumstances, unless a speaker being considered is known to have an impedance curve that does not vary significantly as a function of frequency, I would not count on it sounding similar with your amplifier to what others may report based on experiences with other amplifiers.

An audition prior to purchase would seem to be even more essential in this case than in most others.

A speaker such as some of the Zu models, however, which in some cases have very high nominal and minimum impedances (e.g. 16 ohms nominal, 10 ohms min), will minimize that concern. Although that kind of impedance characteristic would introduce a different concern, which is that your amp does not provide a 16 ohm output tap. Using an 8 ohm tap with a 16 ohm speaker may reduce the amp’s power capability and increase its distortion, perhaps not greatly but the degree is hard to predict.

So I would add to Doug’s good suggestion of an 8 ohm speaker having 92 or 93 db sensitivity, or thereabouts, that it would be highly desirable for the speaker to have as little variation in its impedance vs. frequency curve as possible, especially if an audition is not possible. Impedance vs. frequency curves are generally included in Stereophile’s speaker reviews. Given the output impedance characteristics of your amp, as a general rule of thumb frequencies at which speaker impedance is high will receive significantly greater emphasis with your amplifier than frequencies at which speaker impedance is low. And, conversely, frequencies at which speaker impedance is low will receive significantly less emphasis with your amplifier than frequencies at which speaker impedance is high.

Good luck in your search. Regards,
-- Al

I have to echo jmcgrogan2 go for the Legacy speakers. There was recently a pair for sale here at the ridiculous low price of $1500.00. What a steal....er, I mean deal!

Regarding the Legacy Focus 20/20, see its impedance curve here:

www.stereophile.com/content/legacy-audio-focus-2020-loudspeaker-measurements

While the nominal impedance is 4 ohms, the actual impedance is 2 ohms in significant parts of the deep bass region, and varies up to about 9 ohms in the mid-range. For the reasons I stated in my post above, I would definitely not expect this speaker to be a good match for the OP’s amp. And in particular, the interaction of the amp’s very high output impedance with the very low impedance of the speaker at deep bass frequencies will result in very weak deep bass. And based on that impedance curve and on the output impedance characteristics of the amp, I would expect that weakness to probably extend up to as high as around 200 Hz.

Also, John Atkinson provides the following comment in the review:

The Legacy’s sensitivity was slightly lower than specified, but still a very high 94.5dB(B)/2.83V/m—this speaker will play very loudly even with a low-powered amplifier. However, its plot of impedance (fig.1) reveals that it needs to be used with an amplifier that can deliver high current into low impedances. Not only are there two minima in the bass of less than 2 ohms, and another of less than 3 ohms in the mid-treble, but there is an amplifier-crushing combination of 3.3 ohms magnitude and 60 degrees capacitive phase angle at 20Hz. Fortunately, it is rare in music to have high levels of energy this low in frequency. However, a good, beefy solid-state amplifier will probably work better than a tube design.

Regards,
-- Al