Tube power amp for monitors? Solid state is also an option.

Most solid state amps approximate a constant-voltage source, which means that they put out more wattage into a low impedance and less wattage into a high impedance. Most loudspeakers are designed with constant-voltage amplifiers in mind.

Tube amps usually approximate a constant-power source, delivering approximately the same power (wattage) across the spectrum as the impedance curve swings up and down, unless the swings are really big and/or go outside the range the amplifier was designed for.

Let me explain how a problem can arise from the different behavior of these two amplifier types (I'll be making some simplifying assumptions but the principles illustrated are correct): Suppose a hypothetical "8-ohm" speaker was designed for solid state. Let's say its impedance curve has a 16 ohm peak at 50 Hz, the impedance dips to 4 ohms at 200 Hz, and there is a 32 ohm peak at 4 kHz. So when our constant-voltage solid state amp is putting out 2.83 volts (1 watt into 8 ohms), it is putting out 1/2 watt into the 16-ohm peak at 50 Hz; 2 watts into that 4-ohm dip at 200 Hz; and only putting 1/4 watt into that 32-ohm peak at 4 kHz. Since the speaker designer expected this, all is well - the speaker sounds balanced.

Now suppose we decide to try a tube amp, which puts out approximately the same wattage across the spectrum. At 50 Hz, the speaker is getting twice as much power as the designer expected (+3 dB), so the bass sounds bloated. At 200 Hz, the speaker is getting one-half as much power as the designer expected (-3 dB ), so the lower midrange region sounds thin. And at 4 kHz, the speaker is getting four times as much power as the designer expected (+6 dB), so the speaker sounds harsh. Our instinct would be to conclude "tube amps suck", but really the culprit was a component mis-match.

It is not too difficult to design a loudspeaker to work well with a low-damping-factor, constant-power-approximating tube amp, as well as with solid state amps. The secret is, make the impedance curve very smooth over most of the spectrum, and use a variable-tuning port system so you can tailor the port tuning to your particular amplifier damping factor/room-induced low-end boundary reinforcement situation.  (With tube amps in particular, amp + speaker + room = a "system".) 

So IF you are going to try a tube amp, unless you know the speakers were designed for tubes, take a look at the impedance curve. Amphions were mentioned - their impedance curves don't swing as wildly as my hypothetical "8 ohm" speaker above, but their nice flat frequency response curves (measured on a constant-voltage amp almost certainly) will be modulated somewhat by the impedance curve when driven by most tube amps.

That being said, even with compatible speakers, I would not suggest relying on a tube-amp-based monitoring system because I think it could potentially sound "too good" and result in a mix that doesn't translate well to lesser systems.  But a good tube-based system just might be killer when it's time to "impress the client".