Hello,
I've tried to put all the reasoning behind my comments a couple of times, from demos, discussions, and owning the Arena, but it ended up too long winded.
The Arena will sound OK on amps at about £400, but I found I got loads more from the sound, especially in terms of clarity and detail, moving up to something like the Marantz SR6001 from amps like the Onkyo 605.
I run my Arena with the SR6001, (about £700 or so when new), and I think it sounds great.
The Arena will keep up with more expensive amps, but I think there are diminishing returns for your money with amps once you break a certain price point.
So I'd say aim for an amp at maybe £700 - 800, or maybe slightly more, and you would probably have the balance point right of doing the speakers justice for the lowest cost.
Though I haven't heard the DG820, I think it is probably a bit too budget for the Arena which was expensive when it first came out.
I'd do some demoing of amps if you can and see what you think.
Hope this helps.