I'm playing through an old VYPR, because I got it for free.  Now that I'm actually playing with people I need to think about getting an amp, but I don't really know what I'm looking for.  Would anyone share recommendations?  In addtion to models, it would be really helpful to understand what makes a particular amp good in your opinion.  What gives it it's unique sound?  Also, I'm not sitting on a $500 bill just waiting to be spent on an amp.  So it would be good to know both what is a good enough set up to get started playing out as well what to think about for the future.

Many thanks,

Bill