Just because I had one handy, I've been using an ART 310a with my Ultra. I have a favorite preset that I've been tweaking for a while, and I like the way it sounds on the 310a. However, if I do something as simple as laying the 310a down as a monitor rather than upright, the sound changes totally (mostly due to reduced bass coupling with the floor) and I don't like the preset at all. It also sounds quite different on my studio monitors, which I couldn't conveniently connect to my Ultra until recently.
Probably I should be dialing in my presets on the studio monitors to start with, instead of counting on some arbitrary coloration from the 310a. Then I need something that is more accurate for performances, whether that means compensating for the response of the 310a or purchasing something else that is more accurate. Fortunately I have a lot of time to educate myself on this, especially since there are a lot of conflicting opinions here on the forum.
I do understand why people like the 310a; it is pretty easy to create presets that sound good with this speaker.