I personally don't find that the case, of the many Senns I've heard(including the cheap stuff), I found Senns often lean on the dark and laid-back side. When it comes to Senns higher-end models, they do tend to lean more towards neutrality though. I found the HD 600 and HD 800 to sound very different from each other. I personally don't think a house sound has anything to do with how dark or bright the headphone manufacturer tends to lean, but rather it's a more subtle character to the sound that is often found among a brand regardless of FR response.
Generally I agree, but as I mentioned, I think the HD600/800s represent the "best" that Sennheiser has put out. But with darker sounding headphones (i.e. HD650s), I think one can't pin down a Sennheiser "house sound"; very much unlike say beyerdynamic.