Owning 2 SMSL's is meaningless if you don't have a Grace to compare them to. You can be perfectly happy in your ignorance of what true transparency sounds like, and if you are I recommend not auditioning an m920. However, no SMSL I have heard (using the same cans, HD800's) has come anywhere close to my m903. You lose transparency, you add noise, the imaging is inferior.
That doesn't mean they are bad, they just aren't anywhere near as good. They are a great value, like I said I run an SMSL at work where my listening is rarely my focus and more just background and I am completely happy with them. But when I am sitting in my chair intently listening, where the listening is my sole focus, like I often do at home it just doesn't get the job done for me anymore.
There are plenty of places in audio where you can spend vast sums of money for relatively small increases in quality, I personally don't think the Grace m9xx series is one of those places, but I also buy my high end gear used mostly to lessen the pain. I got my m903 for ~$500, and at that price point there is zero chance of finding anything else in the same galaxy when it comes to sound and build quality, imo. At $1400 it gets a bit tougher, but for anyone with the disposable cash to spend they won't be disappointed.