If your wanting to spend 300, I would look into TUBE POWER.
The standard answer here is " Go to the store and play with all of them."
How much more sound do you want? I cant remember but does the cube have an external speaker jack? You could just pipe it into a 10 or 12 in speaker. That amp is designed for low level playing and pratice. If you want to play with a drummer,, Im thinkning at least a 15 to 30 watt amp. I held my own back ing the day with a little Marshall lead 12 amp.
A 5 watt tube amp = around 30 watts of solid state. If you have a multi effects, you really dont need a modeling amp, if its economy time,, look into Berhinger, or Bugera http://www.bugera-amps.com/EN/products/V5.aspx I have this one and a 65 Watt berhinger 2 channel (both the same company) Go play in the store and get what makes you feel good.
Mal - Well, lady, I must say, you're my kinda stupid.
Mal - Jayne, your mouth is talking. You might wanna look to that
Kaylee - No power in the verse can stop me. BOOK- you're going to burn in a very special level of hell. A level they reserve for child molesters and people who talk at the theatre.