Try
this.
From what I read the power rating for that KH at 4 ohms is 150 watts. The UPA 2 is gone forever unless you can find one used. Is your question above regarding a pair of UPA-1's?
The calculator linked above provides this:
At 3 meters, 150 watts will get you 100.22 db with no headroom.
At 3 meters, 350 watts will get you 103.9 db with no headroom.
350 is the rating for the UPA-1 at 4 ohms.
I went from 130 watts at 8 in my Onkyo to ~260 in the XPA-2. My speakers are 88 db sensitive. Their impedance is probably closer to a nominal 6 than 8, so the actual wattage provided by both the Onkyo and the XPA-2 might vary from the above numbers. There is no doubt a noticeable difference at the upper volume levels.
I could definitely notice the sound start to come apart at higher listening levels with the Onkyo. I have yet to experience that with the XPA-2 and I have tried. My ears didn't thank me.
The benefit in the power jump is there, but I have to crank it to get there. The degree to which you will benefit is a factor of your listening levels, your distance from the speakers, and the dynamic of your music.