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

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. 

I can only change frequency. Slope is 18db/octave. I don't know Bessel, ect... You are correct on the crossover frequencies except I thought it was 140hz not 125hz.

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?

I have a couple. I have 2 audio control units from a Richter scale and an Epicenter. I can swap crossover frequencies easily. I also have an Audio Research tube crossover  ec-4 which is hard to change. I think it is 12db on bass and 6db on mid and high. I am using that on a set of Tympani 1D but would swap it out if it was a better choice. And lastly I have a Nakamichi ec-302 which is a 12 volt unit on a power supply. The Nak can select 10hz steps so it is great to experiment with.
Marc