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.