What can be a little nebulous and easy to forget about is that many solid state subwoofer plate amps are 10K, so that's the kind of thing you'd want to watch out for!
There are also main power amp designs from various manufacturers with low input impedance - a few examples would be First Watt F1J, Pass Labs Aleph, Bryston 28B SST2, Aragon Iridum. This is possibly due to temporary insanity of the designer, as it is typically one random model out of the manufacturer's product line that has this low input impedance. So it's worth looking up the input impedance of whatever amp you have when looking for a preamp to go with it.
I had to match up a Foreplay III with a Pass Labs Aleph many years ago at CES and there were several phone calls to PJ to work out the necessary changes along with a few runs to Radio Shack and a lot of field expedient soldering in the exhibit room to accommodate the abnormally low input impedance of the Aleph. Luckily those were the days when I hauled a rolling tool case with me to shows. Nothing like a little pressure to make a trade show fun. If still did shows now I would haul PB with me instead.
