How to accurately gauge speaker sensitivity to match with tube amp?

I recently purchased a pair of floor-standing Revel 328s (sensitivity 91dB, recommended power 50-300W).  I'm driving them with a Raven Nighthawk tube amp (20 wpc, high current IA).  Question: Am I harming the speakers by underpowering them?

@nakam Yes you can mess up the tweeter if you drive the amp to clipping. The recommended amp range is an estimate the speaker company uses as a guide for consumers as long as you don't push the amp which means listening for clipping even in moderate listening levels at the transients.

When the amp clips the current doesn't alternate but behaves like DC which the capacitor in the crossover will pass,  especially bad for tweeters.

@nakam @djones51

When the amp clips the current doesn’t alternate but behaves like DC which the capacitor in the crossover will pass, especially bad for tweeters

While I of course agree that overdriving an amp to the point of clipping, or at least severe clipping, can and will often damage tweeters, and while many audiophiles apparently believe the quoted explanation, that explanation is not correct.

Capacitors are commonly used to **block** DC. Consider for example the coupling capacitors that are used at the outputs of many tube-based preamps or other line-level components, or that are used to couple signals between active stages within those components. They are there to block DC, perhaps among other reasons depending on the particular design. Also, the impedance presented by a capacitor **increases** as frequency decreases, and DC is zero Hertz.

Following is an explanation of why a clipped amp can damage tweeters, quoted from a post I made in a thread back in 2011:

The reason that speakers are commonly damaged by clipping of underpowered amplifiers is essentially as follows: Clipping occurs when the amp is asked to deliver a larger voltage swing at any instant of time than it is capable of delivering. That will usually occur at bass or mid-bass frequencies, where music typically contains its highest energy levels. Clipping means that the positive and negative peaks of what would normally be a smoothly varying waveform will abruptly transition to a flat, essentially constant output level corresponding to the maximum positive and negative voltages that the amp is capable of delivering.

The ABRUPTNESS of those transition points corresponds to high frequency spectral components being present in the output signal that are not present in the original waveform. In other words, the clipped waveform contains excess high frequency energy, which the speaker’s crossover will duly route to the tweeter. That can occur even if there is no high frequency energy at all in the original signal, as a result of the clipping of bass frequencies. Tweeters are ordinarily able to handle much less energy than lower frequency drivers, and can therefore be damaged by that excess energy.

Solid state amplifiers will typically clip more abruptly than tube amplifiers, therefore making them more likely to cause that kind of damage. Although if a tube amp is clipped severely enough, the same damage can result.

Severe clipping will be immediately obvious, because the sound will be horribly distorted. The onset of clipping may be characterized by mild distortion or slight popping sounds on musical peaks. I doubt that occasional mild clipping would cause any damage.

@nakam, with regard to your specific question note particularly the last paragraph above.

Regards,
-- Al

@nakam
20wpc is good for 96db @ 3.5m w 91db 2.83V/8Ω/1m speakers
Plenty loud.

Speaker impedance may change the above somewhat