A lot of Wagner's music that is FROM opera doesn't necessary sound like opera, so I think you're safe.
In fact, the overtures and finale's to many of Wagner's opera are played AS concert pieces, as seen in the Death and Funeral March I have below.
If you listen to this first it is a call played by the actor on the stage, but the real hornist sneaks backstage to play it....
You might now have a
slightly better understanding of this, since it has to do with Siegfriend, it involves some of the same material in the climax. This is kinda funny because Mr. Tennstedt's (London Phil's conductor at the time) stand falls at 5:49, sounding like a gunshot, but he really keeps his cool about it. Too bad we don't get to hear the Wagner tubas out loud and independent, they're just in the thick of things.
I would recommend you watch the whole
Der Ring des Nibelungen, which is in four parts:
Those links will lead you to Wikipedia articles on them if you care. I haven't heard or seen the whole thing myself, but I have seen bits of it on YouTube, and if you're careful in searching, you might be able to piece together all the parts, because I think they are all there somewhere.
Most of Wagner is opera, but he DID write a few pieces for orchestra, including a symphony I know nothing about, and a small handful of piano pieces including a sonata or two that never caught on. Here's what I could find of his orchestral music on YouTube:
Somewhat poor recording, sorry.