There's no telling if it would be the same. Is it adequate? probably. I've also wondered the same about my receiver. But I only had the receiver part with no way to play cd's. So for a piece of mind and the ability to hook an amp up to my laptop, I gave in to buying a portable amp.
For a short time I had the dt990 600 ohm. My lower tier receiver(Sony str dh 520) was able to get them very loud with the volume not even turned up all the way.