I'm thinking about going to a pre-amp/power amp set up, but there are a few things about power amps I don't understand, since I've used "ordinary" bass amps all these years. First: On a modern, solid-state stereo power amp that's rated for 8, 4, and 2 ohms, can I run one channel at 8 ohms and one channel at 4? Second: Can I use just one channel, if I'm using just one cab? Third, I don't really need much power at this point, so I was thinking of something in the 600-watt (at 4 ohms bridged or 250 - 300 per channel) range, just to have lots of headroom and room for louder settings. In this category, Carvin and Peavey seem pretty inexpensive, followed by Crest, Mackie and others. Since I don't expect to go anywhere near the limits of the amps, is one brand preferable to another? Also, there seem to be plenty of used 300 watt and up power amps available from the above listed companies and others; is there something I need to avoid or look out for? Thanks.