The BX500's first run had some issues but my understanding is that has long since been remedied. I use a BX1500 and it has performed flawlessly for me for the last 7 months or so. Oh and just for the record the tube in the preamp will NOT get you SVT growl. It does add a bit more flavor but Carvins bass amps are more in line with the "Hi-Fi" market than the "Classic" tube amps market. The reason I went with the 1500 over the 500 was more available power (duh) and a little more versatility with biamping and 3 band mid sweep, verses the 500's two band. It's the little "just in case I need it later" features of the 1500 that pushed me in that direction. Tonally they are identical.
If you have any questions about their amps, Carvin has a pretty active forum community, and although there is a little bit of a fanboyism to it at times, the folks are level headed enough for decent comparisons and reviews. There is a video up on the Carvin Channel (
www.carvinchannel.com) with Phil Martin and Doug Williams playing through the BX500 and BRX2x10 cab.
I like my amp and have no complaints. If you have any other questions about the gear just send me a PM. I am a fan, but not a die hard fanboy. Their stuff just works for me, but if an SVT is better for what you want I'd be the first to tell you to go get the Ampeg.