The Studio 60s need a lot of current, plain and simple. No other way around it. If you're looking for a warmer and smoother presentation, an NAD 375/372/370 integrated can drive them pretty well without breaking the bank. NAD seperates may work a bit better, albeit at a higher cost.
I really like Rotel's seperates with them, but that'll give a bit leaner and brighter sound than I think you're looking for.
The Plinius gear as mentioned should sound fantastic too.
Again, I don't think it's the speakers themselves that ate causing the issue, and it seems like you've played with placement too. I'm very confident it's due to the Denon running out of gas. When amps ate pushed too hard (low impedance in this case), the sound gets pretty shrill among other things.
I really like Rotel's seperates with them, but that'll give a bit leaner and brighter sound than I think you're looking for.
The Plinius gear as mentioned should sound fantastic too.
Again, I don't think it's the speakers themselves that ate causing the issue, and it seems like you've played with placement too. I'm very confident it's due to the Denon running out of gas. When amps ate pushed too hard (low impedance in this case), the sound gets pretty shrill among other things.