There is a fix for you 9.0 was the 9.1 changed the crossover or some such thing. Today Class D amps will push those EASY. Hypex or Ice.
I prefer the Hypex modules with swapable buffer boards. Second the caps in those thing were CHEAP even for the time.
I've tinkered with 8.0 and RS1B, RS2B, RS2.5s worked on all the XOs
The 1bs had factory upgraded servo bass systems.
All of the ones lest the 8.0 kappa, were EASY loads in comparison
The X.O series were know amp killers because the DIP BELOW 1 ohm
BELOW. Ok! A Class D, Krells, Mcintosh. They work, there are a few more. But for the money Class Ds and hunt for the resistor that causes the problem. Look at the 9.1 or 2 schemo. That is how I fixed the 8.0s
I think it was remove a resistor and add a different value, 20.00 fix.
Even simpler yet, imhififan post a good idea too. disconnect the bass speakers and wire them to a reasonable, 2-8 ohms. Lower normally = higher THD, ay A LOT MORE. like 5 times as much from 1 ohm to 4 ohm nominal. PE has a DSP 1200, I use, with great result. More expensive than the 20.00 resistor change, or inductor.. Can't remember, 15 years ago.. BUT there is a fix. Nice sounding speakers, I thought.. Love the old RS series Infinities, I still have quit a few pairs here and there.LOL