It's really more an issue of impedance -- nominal, and how flat or "friendly" the impedance curve is.
The Sapphires are nominal 6 ohms, minum 4.5, and very flat, low reactance, and 85.5 dB
You get more power out of these amps at 4 ohms than at 8 ohms, but you don't want to dip much below 4, and conversely, if you have speakers that climb to 16 or 20 ohms at various points at certain frequencies, count on a lot less available power at those points.
I don't know offhand what the impedance curve of the Rubys is, but as long as it doesn't go too low, or too high, and is relatively flat, it should do fine, but may not besuited for concert level reproduction.
-- Jim