As I said earlier, in my humble opinion, I would keep the Sundara, which you already like, and upgrade from the Denon to a new DAC/amp. Get used to the sound of that combination, decide what you like and dislike, and that will make it easier to figure out later on if you need to upgrade the Sundara. You are likely to keep the new DAC/amp for a very very long time, which will allow you to either focus on upgrading headphones, or if you decide you’re happy with what you have, you might just stop. (I say this theoretically because I don’t appear able to “just stop”).
You mentioned that you might spend up to $2000, so if you have that kind of budget, I would consider one of these options (in no particular order):
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RME ADI-2 DAC (normally $1149 but appears to be on sale today for $919). This is a one box (DAC and amp) solution that can decode DSD from your PC if that’s really important to you. It’s unlikely, but if you ever buy headphones in the future that are difficult to drive, you can always supplement this with an extra amp. Read this very interesting thread for more info. At the beginning, Torq and others rave about the DAC and say the amp isn’t quite as good, but later on, it appears that RME upgraded the amp, and Torq has recommended it as an all-in-one solution. B&H have a 30 day return policy.
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Schiit Bifrost 2 DAC and Asgard 3 amp ($900). For the same price, I’m guessing from what you’ve said that this combo will give you the “emotional” sound signature you’re looking for. I have Bifrost 2 and Lyr 3, which is highly musical but more expensive, but I think swapping out the Lyr 3 for the Asgard 3 amp will suit you very well for rock. The only downside is that the Bifrost 2 doesn’t process DSD, so you would have to continue to use the Oppo to feed it PSM. Schiit has a 15 day return policy, with a small restocking fee.
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Schiit Modius DAC and Asgard 3 amp ($400). If you want something a lot cheaper than the above, then this combo works together well, based on reviews (I haven’t heard them myself but others on the forum have). Remember that hi fi has diminishing returns as the price rises, so just because the above 2 choices are twice are expensive, they are not twice as good.
I mentioned 2 Schiit combos because I’m familiar with their products, and I’ve been researching the Asgard as something I might buy for my son. I’m sure others will supplement the list with other manufacturers that are just as good as Schiit.