good question! The specs for the XA30.5 are pretty good (not surprising since it's a Pass Labs product):
Description: Solid-state stereo power amplifier. Inputs: 1 pair unbalanced (RCA), 1 pair balanced (XLR). Outputs: 2 pairs binding posts. Rated power output: 30W into 8 ohms, 60W into 4 ohms (both 14.8dBW). Maximum output voltage: ±35V. Maximum output current: ±20A. Voltage gain: 26dB. Input sensitivity: 0.77V at 26dB gain. Frequency response: 1.5Hz–100kHz, –3dB at 1.5Hz, –2dB at 100kHz. Signal/noise: 150dB at full power. Distortion: 0.01% at 3W, 0.1% at 30W. Input impedance: 30k ohms balanced, 15k ohms unbalanced. Damping factor: 150 at 8 ohms. Slew rate: ±50V/µs. Power consumption: 238W.
Read more at http://www.stereophile.com/content/pass-labs-xa305-power-amplifier-specifications#pFvKLDoXoFb0JoE8.9...
with it's ability to output +/- 20A my calculations show that it should be able to double in power for each halving of the speaker load impedance (I expected nothing less from Pass who's been a major contributor to writing the book on how to make a class-A amp).
But, let's look at the Revel Ultima Salon II impedance & phase plots (Fig1 in the link below):
http://www.stereophile.com/content/revel-ultima-salon2-loudspeaker-measurements#fm6YgtoIdAOUp14u.97IMO a tough impedance & phase plot - large swings in phase swinging from capacitive in the deep bass to inductive in the upper bass & lower midrange, rather low impedance (4 Ohms) in the bass. This is a hard speaker to drive for any amp. Definitely bring a high current amp to this party (which the XA30.5 seems to be).
The thing to remember is that with non-zero phase angles, the impedance is actually much lower than what is plotted. For example, at 50Hz, the phase angle is approx -20 degrees. My calculations show that the resistive part of the speaker impedance (which is the part used by the amp to create a voltage to drive the speaker cone) is actually 3.57 Ohms (not 3.8 Ohms as plotted in Fig 1). Having a reactive component to the speaker impedance exacerbates speaker impedance i.e. makes a low impedance appear lower to the amp. Thus, the amp struggles more as it has to dump current into a (further) lower impedance.
For a moment forget the brand & model of your power amp.
A 30W power amp (no matter which brand) can output 1.9A into 8 Ohms.
A 60W power amp can output 3.8A into 4 Ohms
A 120W power amp can output 7.75A into 2 Ohms.
All of the above ASSUMING the output voltage does not violate the max output voltage capability of the amp + the AC power xformer is capable of delivering this sort of current.
Next, how much power do you need to generate the SPL at your listening position?
Assume you listen at 3m (10') from the speakers.
The Ultima Salon II is 86dB/2.83V/1m from the Stereophile review.
Thus at 3m the SPL would be 86dB - 6dB - 3dB = 77dB. This includes the detrimental effects of sound absorption due to walls, furniture, drapes, etc + the positive effects of having stereo playback. Essentially these 2 effects wash out.
So, 1W gives you 77dB SPL at your 10' away listening position
10W will give you 87dB SPL at 10'
100W will give you 97dB SPL at 10'
200W will give you 100dB SPL at 10'
400W will give you 103dB SPL at 10'
103dB SPL for transients is not very loud - it's decent tho' but on the lower side. If you want your listening experience to be realistic & you want to get the feeling that the music flows effortlessly, you are going to need atleast 400W of power.
With the Ultima Salon II having an impedance in the 3 Ohms range your XA30.5 is going to operate like a 70-80W power amp. If you reference this to the above SPL table you'll see that you'll get an approx 97dB SPL max out of it. This is not going to be very loud nor realistic. It's going to feel that the amp is reigned in & that the music doesn't have that sparkle or that effortless flow.
For this speaker if one wants realistic levels of SPL for the transients then power begins to increase rapidly, as you can see.
That's why the XA30.5 is not a suitable amp for the Revel Ultima Salon II speakers.
Hope this helps. FWIW.