Why are the Infinity IRS, Reference 1b, Beta ect speakers passive on the mids and highs?

140Hz does seem a little low for the EMIM’s to go down to

+1 agree you could kill them, I'd say more like 200 to 250hz

Cheers George 

The pots were poor.

Pots are always a problem when old in any speaker, you replace the pots with appropriate resistor L pad power resistors of the same values for your preferred setting.

Cheers George

The quality of the passive xovers in those higher end Infinity’s were beyond reproach, and for the mids and highs far better than listening through the half a dozen opamps per channel of an active xover.

But for the bass, yes by all means use an active xover as then it can be direct coupled, which is always better for bass.

Cheers George

Why are the Infinity IRS, Reference 1b, Beta ect speakers passive on the mids and highs?

I think they found as I did. (with both ways bi-amped)
I found good passive 18db xovers on the upper bass/ mids and highs of my Martin Logan ESL Monoliths III sounds more natural than any active crossover I tried, they all had a sterility to sound.
But on the 12" bass 150hz down I use and my own 24db active.

Cheers George