I'm using them in that way right now with a Grant Fidelity TubeDAC 11 and think it makes a noticeable jump switching the power off. Instruments have more separation, feels a little airier in the top-end, and goes a lot louder. After a brief listen, I'd say it is a flatter response with a little less in the low end, and more in the upper-mids and top. Guessing this might be the result of some default EQ that may be applied, or simply the effect of the onboard amps.
I like it. Really nice to have the option there for a slightly more detailed, flatter response with the power off, or the more signature warm sound that the onboard amps and processing give.