Sorry for the late reply!
The main impetus was to move everything to my desktop as I was forced to downsize in order to accommodate my 2yo son and his ever-growing needs. However, the STA9/DAC9 combo is definitely smoother. I'm sure everyone is calling that combo smooth. The A23 was slightly more analytical, and wouldn't hide harshness if the recording contained frequencies that were harsh to my ears. I'd consider the A23 rather flat in response, so when you feed it excellent material, it was astoundingly good. The A23 is all most ppl will need within a medium sized room IMHO. It's an excellent amplifier.
Regarding the DAC9/STA9 combo, it's drier, softer, and the soundstage is slightly narrow. However, I can listen to any genre and it doesn't offend, while at the same time, the timbre of instruments (big deal to me) is still quite good. I'm not sure which filter they chose when they implemented the 4490 in the DAC9, but it seems to be solid all around.
My main listening habits lean towards stuff like Dave Matthews, Sting, John Mayer, Alterbridge, SRV, Al DiMeola, Dream Theater, Joe Bonamassa, Dave Weckl, Tim McGraw, Second Nature, and some random Classical. So, anything from Metal to Country and a lot in between. My speakers are low end Dynaudio m10s, spaced about 4.5 feet wide and listening within somewhat of a triangle (I listen a little further back than they are wide). Room treatment is OC 703 (2" and 4") and Roxul RB 80 2". The room is about 8' deep, 8' high and 15' wide.
This is my lowest cost system by far, though it's still gratifying. I'm working on a custom 8" sub build with a MiniDSP ICEpower amp to reinforce 40-50Hz, but that's the extent of it.
I hope this helps!
Really interested in your conclusions regarding this change! What made you go for the STA-9 over the A23?
I am always reading about equipment and looking for honest unbiased views on different equipments, which is really hard to achieve with reviews that have interests in the results of the reviews. The A23 seems like a nice and balanced Amp, at a comparable price to the STA-9.
I dream of sometime soon changing my integrated DAC-Amp into two dedicated units, and this are interesting choices. So I am looking forward to picking your brain as to why you decided to go for the 9 over theA23 and your results!