Their solid state power stage regulation has a tendency to fail, and once it does, the amp will never be stable into a four ohm load again. Not every one of them does it, but once it goes, it can't ever be completely fixed without replacing the whole mother board. I was a warranty service provider for SLM and got all the parts and printed schematics directly from them when mine died. After two new sets of output transistors, it worked, but not well enough to suit me.
I traded it for an SVT cabinet to a local music store, fully disclosing its condition. I saw it back there for sale several years later, still running after not having been serviced any more (I marked it,) but all I had to do was put it on a four ohm cabinet and it started acting exactly as it had before it died the first time. So if you're going to take it easy on volume and not use a four ohm load, it's fine. Just don't expect anywhere near the volume that its "450" watt rating implies.
