I have a pair of SX-500 Mono Blocks as well. They are not fully broken in yet.
(DO PAY ATTENTION TO ADEQUATE BEAK IN) they have significantly improved over time. Thus far, their wonderful attributes are great bass control, huge deep sounstage, beautiful midrange, good pace and rhythm. Compared to MUCH MUCH more expensive, Audio Research Tube Monoblocs they lack a couple of subtle attributes.... a slight diminution of very high frequency overtones, and
the images of singers are slightly less 3 dimensional (one of tubed electronics true blessings). On the whole, for $1800 these amps are GIANT KILLERS. None of the solid state nasties. I do suspect though, that these amps really need a somewhat warm sounding tubed pre-amp (cary, conrad johnson EAR as well as many others) to really shine. I use an EAR full function pre-amp, the 864
with my SX 500s. A full review to be posted on this site when break in of completed.
Jim