I have difficulty putting what I hear to words, I will say they both have great soundstage depth and width. Both have holographic imaging, which to me means solid and well defined instruments and voices placed within the soundstage, with lifelike space between them. The Valhalla has more drive, sounds more powerful and energetic. I prefer the 300B much of the time for it's delicacy and nuance, how it brings out subtle details that I'm aware of in some of the music I listen to. I think which one a person would prefer depends a lot on what they listen to. I could list some jazz tracks that are good examples but the music won't appeal to a lot of people and they won't get what I'm talking about.
At Pacific Audio Fest I was in the Spatial room early on Friday and Saturday with the X4s and the DS 300B mono blocks. There were two Spatial people (not Clayton or Cloud) plus Don Sachs. They were accommodating and gracious, and it was just wonderful talking to them. Don let me play a couple favorite tracks. There were about four other guys like me in there for a listen. I could tell they were not really into the music I played. Next one of the other guys asked to hear an Alice Coltrane track, I for sure got into that, but it was very different than what I played. Next Don wanted us to hear a track from Air Hadouk Trio which sounded amazing to me, many subtleties, filled with tone and texture. Way different than what I usually listen to but just full of the qualities I'm talking about. I'm listening to that album now and I really like a lot of the tracks.
Guess I veered off and didn't answer your question with the second paragraph. Hopefully you find it interesting and get some insight into what I'm talking about.