Preamp output impedance, Amp input impedance and drive ratio? Pass Labs XA25

Generally you want the input impedance of the amp to be at least 10x the output impedance of the preamp, but as with speakers impedance varies across the frequency spectrum so I look for at least 20x just to have some cushion.  As others mentioned, at 47kOhms you should be absolutely fine with most preamps.  Hope this helps. 

You've gone a little nuts.  47k input impedance is an excellent value, common in modern solid state amplifiers.  It's high enough that you can pretty much pick any preamp at all without worrying about an impedance mismatch. 

At say 4k, it would be an amplifier that you'd have to carefully match to avoid such issues.  

It doesn't mean anything more than that.  

Conversely, a preamp with very low output impedance can drive almost any amplifier without degrading the sound as the volume changes. 

thank you guys!!

Where you might have an issue is gain. The XA25 is spec’d at 20dB gain, which is on the low side. With your Placette passive, and maybe with some of the others you listed, it’s possible you’ll find things a bit anemic.