I just brought home a pair of RS-IIs and the seller (who bought them new in ~1981) happened to have two documents I have not seen anywhere in the Interwebs: A Lab Test published in some Canadian Buyer's Guide, and A Preliminary Spec Sheet for RS-II. I assume the Lab Test is not well known since it seems to be Canadian publication.
The Lab Test article is in particular quite interesting, a multi-page review article w/ full measurements including sensitivity, and full impedance curve. The Infinity RS-II sensitivity is 85.6 db at 1W power at 1m. The impedance varies between 2 and 8 Ohm, dropping down to 3 Ohm around 200 Hz, then climbs up to 8 Ohm at 1 kHz, then dropping down to 2 Ohm at around 6 kHz, and staying between 2 and 3 Ohm from there up to 20 kHz. The author writes that these speakers should be considered as 3 Ohm speakers (and not 4 Ohm as per Infinity's official specs).
So based on the above it indeed looks like these speakers need gobs of clean current power at low impedance loads down to 2 Ohm (the seller originally had a high end Perreaux amp feeding these speakers clean 400 W at 4 Ohms!, but I unfortunately did not have a chance to hear that combo). And the need for plenty power is amplified even more by the very low sensitivity/efficiency of these speakers, meaning that one has to crank up the volume more than usual for the same perceived loudness. I have already noticed this w/ my new RS-II's in comparison to RS-IIIa's, where RS-IIIa's are clearly much louder at the same volume setting.
Here's the lab test part 1:
http://www.audiokarma.org/forums/attachment.php?attachmentid=549312&stc=1&d=1414368695
Lab test part 2:
http://www.audiokarma.org/forums/attachment.php?attachmentid=549313&stc=1&d=1414368695
I had to split the above in parts due to file size limitations here on AK.
Preliminary spec sheet:
http://www.audiokarma.org/forums/attachment.php?attachmentid=549314&stc=1&d=1414368695
Anyways, hopefully some of you find these useful. I was _very_ excited to find the Lab Test article.