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

No, Passive is not better.... I think the real issue is that these are all older speakers.  When these came out, small amplifiers were not abundant as they are today.  At Marcof, we were experimenting with building chip amps for just this purpose. If Infinity revamped these today, I would not be surprised to see the drivers individually crossed and amped.

Hi Marc, Well, I can at least speak to the RS1B.... On the top end,  we found that carefully matching the sensitivity of parts and fixing their output rather than using the pots was noticeable improvement. We never really changed the design, but a few key parts changes also made a difference. No doubt, they were well thought out...Not sure about beyond reproach.  I'd rather listen to them matched with a good electronic crossover and amp any day. No comparison. If anyone truly multi amped these speakers with quality product, then take the time to properly adjust them out, t I would be surprised if you liked the passive crossovers better.

@bdp24 
"The Emit and Emin drivers were good, but the way they were employed left something to be desired; different frequency ranges appeared at different heights, for one thing".
This is corrected when you hand match the drivers for exact output and hardwired the pad. The pots were poor.

I have not triamped these, I've never owned them.  I worked for Marcof/SpeakerCraft, we did alot of custom work and I got these to work on years later because of my time there. I worked on the crossovers only... Measured each ribbons output individually, then customized the pads within the crossover.  So without having complete driver measurements, that's why the basic design wasn't changed... better caps and coil or 2 and hand matched pads. 
For me, as long as your amps can handle driving ribbons (some cannot) and you have a very good crossover, this is a no brainer. 
It will take a bit to get these matched, but I would expect excellent results. 

I'm going by memory, I could be wrong... What crossover are you using, I'll look it up.

meaning which electronic crossover are you using?

One of my best friends on the planet is an old timer very respected figure in the audio world.  He uses a 12v crossover with a passive sub, of course, he modded it with some nice parts and built a fully regulated 12v supply, but it does a great job.  Years gone by, I worked for Orion. I've heard of few of their amps in home use and at one time I owned a stereo store,  we did 12v to high end...and there,  we tried a soundstream amp with some home gear... I've seen very respectable results mixing.... Never true hard core audiophile with the amps, but some truly respectable results.

If I recall correctly,  they are at 125, 700, 3k & 8k....
Does your crossover have a choice of Bessel, Butterworth, Linkwitz Riley etc?  Can you change slope? Meaning 12db per octave, 18, 24...or even 64db per octave? 
Please let me know... type of slope and amount of roll off effect phasing and blending, plus each type of slope sounds different.
Once I get that, I'll give you some basic advice to try. 

Hi Marc,
   Ok, I'm familiar with most of what you have... The Richter Scale is not really adjustable enough, I was made for a fairly low crossover point for a 2 way with an eq on top.  The Epicenter and the Nakamichi are 12v products...  The Nak would probably work fairly well. It one of the better units for car audio.  As much as I'd like to see you use the Audio Research the 6db slopes on top will not work well with the ribbons.  
There are more slopes than besel, butterworth and linkwitz riley, but these are the main 3 used today. The act differently with how linear they are and how they handle phase. Depending on frequency, 1 can cause smearing, another will bring a rise at the crossover point and another will be flat at the crossover...
Being able to mix slope, we can correct some errors... Mix as in 12 up and 18 down... Although most electronic crossovers today are linkwitz riley 24/24 and that is always a good choice... even though you may like the sound of another slope, when in doubt, Linkwitz Riley is the go to...
So the Nak is 18db per octave and I don't know what type of slope it uses.... but you can't make any changes, so it is what it is.  The Nak will allow you to experiment, but if you ever want to get the most that you can from these beautiful speakers, you'll need something more versatile.
What does it sound like using the published crossover points of 700 & 8k?  and of course you realize the real problem comes in level matching. 

I thought that I'd stop speculating and look these up:

Here are the spec's from the Stereophile review:
Description: Five-way, two-chassis, dynamic, floorstanding loudspeaker system with four EMIT ribbon tweeters, seven EMIM ribbon midrange units, and six servo-controlled polyproylene-cone woofers. Electronic crossover frequencies: 125Hz, 700Hz, 3kHz, and 8kHz.
Dimensions: LF section: 60" H by 14" W by 15" D. HF section. 60" H by 22" by 13" D.
Price: $5295/system (1985); no longer available (2006).

Read more at http://www.stereophile.com/content/infinity-reference-standard-1b-loudspeaker-specifications#3FRfKtT...

and here is the owners manual: 
   http://www.davidsaudio.com/rs1b-manual.pdf

Oh Marc,  I haven't had a stereo store in 30 years.... even though I had a good run in 12v.  My roots have always been in the Audiophile world.