Speakers that are good with tubes

With high powered tube amps, I don't think there are many restrictions on what speakers can be used with the amp.  The issue with low impedance speakers has to do with the interaction between the amp output impedance and the impedance of the speaker affecting the frequency response.  It is NOT a safety issue because tube amps can safely run into very low impedances (even a dead short).  If a 4 ohm rated speaker remained at 4 ohms across the entire frequency band, there would be no change in the frequency response resulting from the output impedance of the amp.  But that is never the case, and the closer the output impedance of the amp is to the impedance of the speaker, the greater the interaction with respect to frequency response.  Tube amps tend to have a higher output impedance than solid state amps meaning that there is more of a tendency to affect the frequency response.  While it is generally better to avoid such interactions and the attendant frequency response changes, sometimes these changes can actually sound good.  Hence, higher and flatter impedance curve speakers tend to be better for use with most tube amps.

For my personal taste, I generally tend to like low-powered tube amps over high-powered tube amps.  There are many high-powered tube amps that sound lean and brittle (hard) to my ears such that if I really required a lot of power, I would go with solid state (the sin of most solid state amps is that they sound dead and unengaging at low volume levels, but this is better than being hard).  Hence, tube friendly, to me, means a speaker is both reasonably efficient and has a reasonably high impedance.

I have not heard your McIntosh amp and I am not familiar with the sound of EL509 tubes, so I can only offer very general advice.   If you know the output impedance of your amp, that might help you decide whether it matters at all if a speaker is "tube friendly."  It the impedance is under one ohm (equivalent to a damping factor of 8), it can probably work well with just about any speaker on the market.  If it has a damping factor above 8, there is even less restrictions.  Below 8, you may have meaningful frequency response swings with lower impedance speakers, such as those rated at 4 ohms.

@pinotnoir , greetings. I too have Soundlab speakers and have been waiting patiently for my pair of Atma-Sphere MA 2s for 6 months. We have a very special situation. Our speakers range from 30 ohms in the bass to less than 1 ohm at 20 kHz. The Atma-Spere amps put out the same power regardless of impedance, perfect for the Soundlabs. Most solid state amps are on their knees at 30 ohms. It takes a real SS bruiser to drive this load well, think 300 watts and up. These amps can really not be compared to "normal" tube amplifiers. 

An amplifier is not supposed to have a "sound." In my experience the very best tube amplifiers and the very best SS amplifiers produce very similar performance with the appropriate load. Given that the speaker/room is the weakest link in just about all systems you choose the speaker that matches your room and your wife best then buy an amp that drives that particular load well. Buying speakers to match your amp is folly. 

Give the Golden Ear Triton 1r a listen. They are 92 db sensitivity, relatively benign impedance curve, and each contain a 1600w amp to handle the bass which makes driving them with tubes very easy. I drive mine with a pair of 50w Cary monoblocks and couldn't be happier. 

J.Chip

I drive my Wilson Maxx 2 with tubes because I prefer the sound to using a very good solid state amp.  Tube amps tend to be lower output power so you need speakers with fairly high sensitivity.

Electrostatic speakers are wonderful if you like them, I have heard dozens driven with every type of amplification.  Sorry I've not heard one I liked. It's purely personal I know however, as more than one HI fi expert and reviewer has observed, "choose which type of fake you like", nothing in your lounge room sounds like the real thing, good even great, particularly to you but still fake.