Amp pairing questions

I’m thinking about purchasing a pre-owned Pass Labs XA30.8 amp (fits within my budget) but have some questions.
My system includes:
Cary Audio SLP70 tube preamp
Chapman T7 speakers (90dB sensitivity, 4ohms impedance)
Will the XA30.8 be compatible with my preamp and speakers?
Now for a question that will surely show my lack of knowledge.
The specs on the XA30.8 state power output at 30W into 8ohms and 60W into 4ohms. My assumption is that the amp detects the impedance of the speakers and produces the output based on that. Is this a correct assumption? 

Showing 2 responses by mlsstl

The amplifier doesn't "detect" anything about the speakers.  Here's how it works for a solid state amp: 

Your Pass amp is capable of a voltage output of about 15.5 volts.  This is true no matter the impedence of the speakers. Using the standard formula, this means that an 8 ohm load draws about 3.9 amps of current, giving the 30 watt figure. 

However, if you halve the impedance to 4 ohms, this doubles the current draw since there is less resistance. This jumps the power to 60 watts. 

The only things standing in the way of this high wattage figure is capability of the power supply in the amp and the ability of the output circuit to withstand a high current load.  At some point the resistance drops so low that the amp sees it as a short circuit and either: 1) runs out of power; 2) engages its self-protection circuit; or 3) fries the amp.  (Tube amps are a different story due to the output transformer.)  

Also remember that speakers don't have a flat impedance. The 4, 6 or 8 ohm figure commonly seen is either a minimum or nominal figure. The actual resistance varies with the frequencies involved.

You didn't ask, but the question of whether this is a suitable amp for you depends on how loud you listen. If you consider "loud" music to be in the 80 to 85 dB range, the Pass's power output should be sufficient. OTOH, if you are a headbanger and like your music at 100 dB plus levels, you're going to run out of steam.  I always recommend people invest in an inexpensive sound level meter so they have a reasonably accurate idea of just how loud they play their music.
I do need to make a correction. The 3.9 amp draw is at 4 ohms. The 8 ohm draw is about half of that, just under 2 amps. Sorry for the mistake -- sometimes I type too fast.