Several posts in this thread have mentioned noise floor and hiss (or lack thereof), so I thought I’d share my experience.
I used to own a dbx (by Harmon) 223s. This is the version with TRS sockets instead of XLR, but otherwise identical to the 223xs. I used it in an unbalanced configuration, with RCA cables and RCA to TRS adapters on the output. (My balanced preamp fed the 223s input using standard balanced cables, 223s HP fed stereo power amp using the unbalanced connection described above, 223s LP fed powered sub also with unbalanced connection.) My speakers were Ascend Cmt-340SE (rated sensitivity 96dB @ 2.83v / 1 meter).
This configuration resulted in audible hiss at normal listening levels. I checked with Harmon tech support, and they agreed that this was to be expected, not the result of a bad unit. They said the hiss wasn’t so much caused by my unbalanced connections on the output side per se, as by the fact that the 223 series is optimized for a +4dbu line level that is typical of pro gear, and not the -10 dbV line level used by my consumer gear. (I am not an EE and don’t know a ton about electronics, but I would have thought the line level is dictated by the input, which was a balanced connection from a balanced preamp). what I took away from them is that the noise floor of the 223s is indeed high.
incidentally, I eventually sold it and replaced it with a Marchand XM66 (variable frequency sibling to the XM44 reviewed by Amir). Whatever the differences in transparency, I got a lot more personal enjoyment using the XM66 in my rig because of its lower noise floor. When I compared the two units side by side and level, subjectively I didn’t perceive any differences in transparency/detail or frequency response/tone, just a blacker background when running through the Marchand unit. For me, it was worth the added cost of the XM66. (When I spoke to Phil Marchand, he recommended the XM66 over the XM44 for its adjustability, but he didn’t mention any specs or measurements to me, so I have no idea what if any performance differences there are between these two models.)