I've used the D.I. several times, it does it's job with no complaints. The Compressor 'sort of' is the gain. It 'plumps' the sound up... it's very clean without the compression engaged and gets more rounded/fatter as you wind it up.
Try one, they're great! I'd definitely start with the 300watt. The 500watt is ridiculous and the compression pushes it further...
I think the 300w are £329 at the moment. I can't think of anything new for that money that can touch it. I guess if it had Mesa Boogie or the like on the front it be all over the the internet.