Actually, I own the Klein+Hummel O300D. The KH 310 is the newly redesigned model. It appears they've made several improvements and reduced the price.
As far as I know, they are designed to be used horizontally and that's how I'm using them. Each speaker is labeled Left or Right and I'm using them that way; the tweeters are on the inside.
The O300D is the best speaker I've owned (and heard). I've bought the last several pairs of speakers without hearing them first and I have not been disappointed. The previous active setup was JBL LSR4328 + LSR4312. The system before that was passive: Bryston BP26 preamp, Bryston 7BSST monoblock amps, Spendor S8e floor standing speakers.
With some speakers I tend to lean forward trying to hear into the recording, listening for details. That's not necessary with the O300D. Yet, the speaker is never tiring. It's as easy to listen to after 4 hours as it was after one.
I had an active system in my office: KRK VXT6 which I think is a tremendous value. A friend owns a pair of Focal Solo 6 and we both agree that the O300D is in a different league. I considered the JBL LSR6300, but I wanted a 3-way design.
I also own the KH 120A. It's the best small speaker I've heard. All speakers with vertically aligned drivers will have less vertical dispersion than horizontal. Thus, you'd think the horizontal dispersion of the O300D would be lacking, but that's not the case. Check out the dispersion plots on the Neumann web site. Then compare that to many expensive passive audiophile speakers on Stereophile's web site.
My only source is CD. Being truly professional recording monitors, I believe KH speakers will let you hear poor recordings as just that.
Hope this helps,
P.S., I use a pair of HSU ULS-15 subwoofers with the O300Ds.