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? 
From the impedance matching point of view, the amp's 50K Ohm (for SE inputs) input impedance should be fine with Cary's 800 Ohm output impedance (probably @1 KHz). However, most folks using the amp seem to think the balanced connection sounds better. I have no personal experience, just reporting what I've read from the owners' feedback. 
1+++ @misstl. Excellent answer. I have not much to add. Sonically the XA30.8 is an excellent amp. It will drive your speakers to a comfortable 95 dB which is as loud as you should go if you like your ears. If you do subwoofers down the line you can increase that number to 105 dB at your own peril. 
Thank you all for the great advice and education. MUCH appreciated. I think I'll go ahead with the XA30.8. I'm certainly not a headbanger person. Mostly jazz and some pop.
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.