I've never owned a Decco65, but from looking at the description page it seems similar in that it's an all-in-one box. What I like about the Grand X-1 is that it has really nice build quality, just feels and looks great. It has truckloads of power. It has a great DAC based on the ESS9018 chip. Their USB implementation is fantastic, asynch with galvanic isolation and XMOS chip - if people could hear the sound I'm getting just running a $10 Belkin USB from my computer to the Grand, there'd be a lot less criticism of USB as a transfer medium.
The X-1 has a nicer tube buffer than the Decco, though I don't use it a lot. If I have a crap recording I'll turn it on, like a couple of days ago I put on some early Pat Benatar which was just a lousy recording, really thin-sounding and strident. I turned on the buffer for that, and it softens the highs and makes it a little warmer and more listenable. But I don't use it a lot because the amp has a nice neutral sound on its own - it's not cold or bright sounding and doesn't need tube "help". If the recording is nice, the music comes out nice.
If you like the excitement of buying separates, and creating the whole combination yourself, you wouldn't buy one. But that's what I was after - one box that made it simple. My system couldn't be easier - computer to Grand X-1 to speakers, that's it.
Best compliment I can pay the X-1 is that I also have an Audio Research VSi55 integrated, and I struggle to tell them apart, other than that the X-1 can play a lot louder.
Objectively, though, at this price level of $3K I don't think you'll make a bad decision. Whether it's the Grand, or a Hegel, or something from Rogue, or W4S or Bel Canto, I don't know that there are any bad answers here. Lots of great choices, I'm sure you'll end up happy.
