My Mirra chair is up for sale. £150
It is a Herman Miller chair so it is really solid, super comfortable and I have loved using it.
I purchased the optional no-arm stem covers and always used it this way which is perfect for playing guitar. The original adjustable armrests (with the required parts) are included and adding them to the chair is a fairly straightforward job.
It is in good condition and everything works well. There is a small cracked section in the middle of the lower part of the backrest. but tis does not affect normal use - it has been used in my studio for years but it still has plenty of life left in it. See photos for details.