The mbls are not that hard to drive with tubes. The issue that you run into is that they are designed for an amplifier that makes constant voltage into any load, i.e. a transistor amplifier.
If the tube amplifier has enough negative feedback, it will be OK. The issue you run into is pronounced output in the 5-9KHz range otherwise. If that happens, its not that hard to set up a Zobel network to take care of it. At that point many amplifiers would suit the bill. Due to the omni pattern of the speaker, they measure as low efficiency, but once in a room, they are not hard to drive at all.
If the tube amplifier has enough negative feedback, it will be OK. The issue you run into is pronounced output in the 5-9KHz range otherwise. If that happens, its not that hard to set up a Zobel network to take care of it. At that point many amplifiers would suit the bill. Due to the omni pattern of the speaker, they measure as low efficiency, but once in a room, they are not hard to drive at all.