This is a tough question to answer, actually. It really comes down to personal taste and what you will be using the amp for (as this will determine your output needs). There are pros and cons for both Tube Amps and Solid State Amps (and you will most likely hear them all on a post like this). Your only real limitation is the amount of money you can spend-there are a ton of choices in this price range. You're not going to get a high end Marshall for that money (you could get a "MG" half-stack for that $600) or you could pick up a Peavey Windsor Half Stack (all tube 100watts, 4X12 cabinet) right now on sale at Musician's Friend for $499.99. You might look into a Bugera (all tube amp heads) Model 1960 or Model 1990 (150watts and 120watts respectively), you could pick from the Peavey "Viper" line of (Combo) amps with trans-tube technology or into the Vox line of amps with the "ValveTronix" technology. The Roland Cube 80XL is a nice little amp with alot of functionality (80watts, one 12" speaker with a number of output options) for around $350-(Sweetwater has some "DEMO" models available right now for $309). The NEW line of Fender Mustang amps (anywhere from 20watts to 150watts and very reasonably priced)-these are "modeling" amps with a little bit more, especially in the way of effects and "presets" and come packaged with recording software etc. These are nice amps. Just about every manufacturer out there makes a 60 to 80 watt amp in the $299 price point.
You're going to hear all kinds of "guff" from people about any and all "specific" amps mentioned in this post. What you really need to do is go to a nearby "Guitar Center" or "Sam Ash" store-someplace that has a wide variety of amps on hand (or in stock) and sit down and play through the ones that interest you until you find the one that YOU like and never mind what everybody else says. Good Luck! Testing and buying new equipment is just another fun part of being a player.
You're more than likely to get a LOT of "feedback" from people regarding the Peavey Windsor and the Bugeras, but believe me, these are perfectly fine amps and you CAN dial in nearly any and every tone you'll ever want or need-you just have to take the time to dial in the sound that you are after. People can recommend all the $600 amp heads in the world, but if that doesn't leave you with enough money to but a speaker cab then what good is it?
