Sounds like you got a winner there, Den. The source and load impedances of any signal combine to affect frequency response and current delivery of any AC signal whether it's at phono, line or speaker voltage level. The MC is fixed source impedance, so it's no surprise that adjusting load impedance has such a huge effect on the sound. You want the lowest impedance you can get without losing any frequency response. Like rollo said, higher load impedance makes it more bright, because more high frequency is getting through with larger impedance ratio. But lower load impedance gives more bass control and drive, because more current can get through with lower impedance ratio. Power transmission is only concerned with current delivery efficiency, so they strive for equal source and load impedance to maximize current efficiency, but they only need frequency response up to 60Hz. As you're discovering, it's a tradeoff.
Of course the Pathos and Ushers are revealing the changes so well too. Maybe you would have heard a little more from the EE on the new system. But tubes have drawbacks as well as advantages. You're livin it.