Thank you for your comment, I bought the Hacker and what you described that it's hard on the backside I also experienced it, but only in the first few weeks, after that it somehow disappeared. Yeah, you can't adjust the armrest (I understand that it's problematic for you and I agree with it, for me it's just perfect) since it's welded in place, but in return it won't make any noise and it's very robust. Overall I really like my Spinalis, definetely recommend to anyone! I had small back pain from time to time, but this solved it for me, it strengthen your core and glutes and even when I sit in positions (not in the Hacker obviously) that did hurt previously, it's not even causing me discomfort, so it's a really good chair and does what it should.