Hi - my 2 cents. I bought a used Hegel 200 as backup for tube mono blocks. I've been very happy with it. It's got a pretty warm sound for solid state....
the upper frequencies are actually just a little rolled off compared to my tube gear, but possibly that's just in my system. Paid around $2500 for it, as I recall. Haven't regretted that choice. This is not to say any of your other candidates wouldn't make you very happy. Plinius was on my short list but I found the Hegel first. I wouldn't be inclined to spend more for just a built in DAC (didn't realize the 300 had one) but others have posted that the 300 beats the 200 on sonics. So if you can afford a used 300, that might be the way to go, though again I personally would not spend more for just a built in DAC since you already have a pretty good one in the the Oppo.
the upper frequencies are actually just a little rolled off compared to my tube gear, but possibly that's just in my system. Paid around $2500 for it, as I recall. Haven't regretted that choice. This is not to say any of your other candidates wouldn't make you very happy. Plinius was on my short list but I found the Hegel first. I wouldn't be inclined to spend more for just a built in DAC (didn't realize the 300 had one) but others have posted that the 300 beats the 200 on sonics. So if you can afford a used 300, that might be the way to go, though again I personally would not spend more for just a built in DAC since you already have a pretty good one in the the Oppo.