I took the leap and am demoing the XA100.8. It's only been a few days but I can make the following observations.
1) Greater reserves. More in line with what the Magnepan 3.71i dual DWMs seem to need. The objective evidence being that the meter's needle moves much less often--and when it does move, moves less severely--suggesting that the XA.8 stay in class A longer than the XA.5. This is not surprising given the specifications (200W vs 241W before leaving class A).
2) Quieter. The XA100.8 has much blacker silent areas. This was mentioned in some the web review, but it is very true. This was surprising as the XA60.5 and 100.5 were very quiet. The improvement is very impressive.
3) Better mid-range. Only on direct comparison of the XA60.5 and XA100.5 was the mid-range of notably less sweet. Female vocals (such acapella) were notably less there. I can't say whether the XA100.8 matches the XA60.5, but the XA100.8 is a dramatic improvement on the XA100.5.
Something like piano keystrokes are very percussive as are snare drums. My non-audiophile wife could easily note the improvement in the quietness, transparency and mid-range.
4) More powerful bass. This is the best bass control I had had on the Magnepan 3.7i and dual DWMs. I imagine this still could be improved to get realistic power, but we're getting closer.
5) Dramatic. The micro-dynamics are much more powerful and quicker. Drum sets, as mentioned above, are powerful and driving.
6) Improved imaging and sound staging. Instruments and vocals on something like the Band from the Big Pink are a bit better staged than the XA100.5. This may very well be a effect of the better quietness and transparency.
Switching from the XA60.5 to the XA100.5, I traded hi-fi separation for integration. Moved from a lead vocalist and a supporting band to a driving band with the vocalist a more integrated. More like what you hear at a live concert vs unrealistic (but beautiful) studio recordings.
The XA100.8 seems to step back closer to the XA60.5. The vocalist is a bit more forward. The band layered behind. A bit more hi-fi than concert. Again this may be an effect of the greater transparency.
7) Less romantic. I have no way to objectively argue this point. It is only a feeling. The XA100.8 may be a bit less musical than the XA100.5. It is very hard to ignore. As many have noted there is a buttery richness to the XA100.5. Warm blanket wonderful. The XA100.8 has traded transparency for the warmth.
8) Ugly box. There is no question that the XA100.5 is beautiful. The face plate, meter. The XA100.8 is much heavier, industrial and traditional hi-fi looking. The heat fins are an improvement, with that said.
I'm still evaluating the two of the next week, but, on balance, I'm leaning toward the XA100.8 given the Magnepan's demands and my preference. I missed the XA60.5 feel that I'm getting a bit of both now. But, to be completely honest, I could have happily lived with the XA60.5, XA100.5 or XA100.8.