Hi Dumie. I have both the HD650 and the RS180. I have no affiliation or financial interest in Sennheiser or its dealers. I also don't have "golden ears" so can only share my impressions. The sound for these two models is in the same ballpark as far as warmth and range, even soundstage (where 650's have a discernible edge). However, the 650's win out in terms of what I call "presence" and articulation/transparency - revealing the individual instruments in a piece of music better. The RS180's are not quite in the same class, but are still an enjoyable listen and, of course, much less expensive. I like the 180's but confess I mainly pick them up when I want to be wireless. The 180's sound a bit more congested/blurred than the 650's. In TV terms, the color seems similar but it is like the difference between 720p and 1080p.
As far as loudness, the 180's go loud enough enough for me (rock music, etc.) but I do not play at high volumes routinely or often. I tried to dial them up as high as they'd go, and would not say they are as loud as other, wired cans I use. The sound began to be a bit distorted at the highest volume.I used an amplifier as well, between the sound source, DAC and base station. At the highest level they were listenable, which is too high for me,but they sounded okay (but, of course, too loud for me). YMMV, and I wouldn't recommend high volumes in any event. But for most listeners, I would say they get loud enough.
Hope this is helpful.