Are you listening to the HE-400 with pleather pads on? While they're not particularly ringy with pleahters, they do sound hollow. With velours they sound more fleshed out.
If you're listening with velours, it could just be the coloration of the midrange. The lower midrange is full and upfront while the upper midrange/lower treble is recessed. Sennheisers are more linear throughout the midrange. I particularly don't find them echo-ey sounding on Velours compared to Sennheisers or other more linear headphones.
Funny you should ask. I got the velours a few days ago and agree. They do help with the tunnel effect. I don't want to blow this out of proportion. While I enjoy the HD650's a bit more on average, they 400's have their place on my desk and do shine in many areas..:) I don't want people reading my posts getting the impression that the 400's are "terrible tunnel y headphones argh!!!"
I have not fallen in love with the sound change with the velour yet. It has helped the tunnel effect but I sort of miss the tonal balance of the pleathers..there could also be a lot of variation in my hearing over the times of day and listening fatigue and all sorts of other things coming into play though.
A couple of days ago I even removed the outer grills as some people mentioned it opened up the sound stage of the 400's a lot. My first reaction was pretty negative. They lost a lot of their presence. Ended up putting the grills back on in about 45 minutes. I may try again though.