Amplifier recommendations for 86 dB Sensitivity Speakers

@mamifero The speakers you have in mind are 86dB and 4 Ohms.

Sometimes its worth considering what this looks like in terms of efficiency rather than sensitivity, since efficiency is stated in Watts rather than Voltage. This is especially important if you have a tube amplifier!

To convert from sensitivity (2.83 Volts/1 meter) to Watts you need to know the impedance of the speaker, which is 4 Ohms. 2.83 Volts is 1 Watt if the load is 8 Ohms (into 8 Ohms sensitivity and efficiency are the same thing), which means into four Ohms its 2 Watts. That's a 3dB difference, which to the ear isn't much but it represents a doubling of amplifier power. Tube amps don't double power when you cut the load impedance in half. 

Another way of looking at that is subtract that 3dB from the sensitivity spec; you arrive at 83dB. 

That is so inefficient that you may have real problems finding a musical sounding amplifier that also has enough power to drive that speaker much past a conversational level. In most rooms this means you'll want something north of 1000 Watts per channel.

If this seems hyperbolic on my part, consider that to double the apparent volume requires an increase of amplifier power by a factor of 10.

@dekay makes a good point! 

Since your amp doesn't double power into 4 Ohms this is a very practical concern. If I were you I'd consider a more efficient speaker as this combination will have the amplifier struggling all the time. If no-one has pointed this out yet, tube amps are less efficient using the 4 Ohm tap and tend to have less bass bandwidth (as well as slightly less power).

IOW I think that speaker with your amp isn't a good idea. 

@mamifero - This remains a math problem wrt having enough power to drive that particular speaker to the desired SPL for your room and listening position.

The post by @atmasphere below explains why I used 83dB in my calculations.  To his point, even my second example of a 200 wpc amplifier that doubles into 4 ohms will only drive those speakers to a SPL of about 100dB at 12 feet.  That calculation does not take into effect a larger than typical room, speaker impedance anomalies, or the desire for greater dynamic headroom so, even at 200/400 wpc you my have barely adequate power.

As a result, there is probably no way you will have satisfactory results driving those speakers with your current amplifier.  I don’t disagree with the 1,000 wpc recommended by @atmasphere, assuming he means into 4 ohms.  IOW, an amp delivering around 400 to 500 wpc into 8 ohms and doubling into 4 ohms, should be about optimal.  The Cary 500MB amps in my second system provide 500/1,000 wpc of power, and the SMc Audio monoblocks in my main system provide 650/1,000+ wpc and do an excellent job of driving Aerial LR5 speakers that have the exact same sensitivity and nominal impedance specs as the Technics speakers you are considering, while dipping to an even lower minimum impedance of 3 ohms. 

As to sound quality, IME, amplifiers do sound different from each other so, like any component, you will need to find amplifiers delivering the type of sound you want to hear. However, if you don’t get the power right, your speakers will never reach their optimal sound quality.

It's like dating a girl and you don't like her parents.  They don't like you but she likes you.

Overall not a good situation probably worth avoiding unless this girl is simply awesome because she's willing to put up with you and it's doubtful you'll find someone better.

@mamifero I forgot some things. 

There is a phenomena known as 'thermal compression' in loudspeakers. Its where the voice coil heats up with bursts of power through it. The more it heats up the more compression.

This is why you read about more efficient speakers being more dynamic- with less efficiency (or sensitivity) the bigger this problem is. Vented pole pieces and the like certainly help, but do not solve that problem.

Here is advice I give to anyone who owns a tube amplifier:

If you want the most performance out of your tube amplifier investment, consider a loudspeaker that is 8 Ohms or more and as efficient as you can get (watch out for speakers with dual woofers which might be 4 Ohm in the bass but 8 in the mids and highs, which might simply be rated as 8 Ohms or '8 Ohm compatible'). 

All amplifiers regardless of technology make more distortion into 4 Ohms and speaker cables become far more critical too. Quite literally if you could make the speaker be 8 Ohms or more it would sound smoother and more detailed due to the reduction in distortion from the amp. This is especially true of tube amps. 

There is one tube amplifier that was designed specifically to handle low impedance loads: the Music Reference RM-200. You can read reviews of the original and Mk.2 iterations of the amp by Michael Fremer on the Stereophile website, with test bench measurements by John Atkinson. Both men were very impressed with the amp, Fremer declaring the amp his choice for best "reasonably" priced tube power amp. The RM-200 remained in Stereophile’s Recommended Components List for over ten years! Each channel of this stereo amp creates over 100 watts from a single pair of KT-88 tubes, and does so without running the tubes hard (and therefore shortening tube life).

An option to consider is to add a real good subwoofer to augment your speakers, with an external active crossover to filter out the very low frequencies from reaching your tube amp. Removing just one octave of bass (20-40Hz, or 25-50Hz, 30-60Hz, 35-70Hz, 40-80Hz, 45-90Hz, 50-100Hz) cuts your power amp requirements in half! Remove two octaves (20-80Hz) and that figure rises to 75%!! But that is true ONLY if you remove those frequencies from the signal reaching the amp powering the speakers.

By the way, for anyone wanting a dipole planar-magnetic loudspeaker to use with a tube power amp, consider the Eminent Technology LFT-8b. While it is as low a sensitivity/efficiency design as are the Magnepan speakers (around 84dB), the LFT-8 presents an almost perfectly resistive 8 ohm load to the power amp. And, since it has dual sets of binding posts (one for the planar-magnetic panels, a second for the 8" dynamic woofer in a sealed enclosure, for frequencies up to 170Hz), the speaker can be easily bi-amped, drastically reducing the power requirements of the panels. Those panels themselves present an 11 ohm load to the amp, great for tubes. The ET LFT-8b got a rave review by Steve Guttenberg, who stated in his review that he considered the speaker superior to not only every Magnepan he has heard, but also every electrostatic design. I’m not sure I would go THAT far.