I have this exact situation. I recently got LR Sierras and I am now wanting to get a sierra center and two side channels. I first looked at the htm-200 but it seemed too large. I have a couch against the wall and limited distance to the side boundaries, so it would seem strange to have a speaker larger than your head so close to you. I also could not many reviews on the htm-200 so I am not sure how it compares to the other, smaller alternatives I am considering.
Looking into the Def Tech Promonitor 800 and the NHT Superzero 2.0. Should timbre matching play a role in the selection? The NHT has a silk dome (vs Def Tech aluminum), which seems like it would be more similar to the Sierras. I do like the htm-200 as it should be the most matched to the Sierras. Does anyone know if you would hear a difference in sweeps and matching?
Looks like the Def Tech could handle more bass, reducing localization issues. It is also a little smaller, but it might not work as well for you with the radiator on top.