I think one of the biggest misconceptions about power (especially when someone is just starting out in this hobby) is that going from 50 watts to 100 watts, while being twice the power will be twice the volume. NOT TRUE. The truth is, to achieve twice the volume you need "10" times the power. That means you need a jump from 50 to 500 watts to play twice as loud.
That was pretty much black and white. Where things get a little gray: Why do some 50 watt amps sound better/louder that some 100 watt amps? This is where the current, power supply, characteristics of a particular speaker, etc come into play. Example: Your speakers are rated at 4 ohm. That's an "average" the manufacturer has given it. It may be 2 ohms at some frequencies and 10 or more at others. Ahendler and I both use Magnepan 3.6's. They are rated at 4 ohms and 85 db but may actually be easier to drive than your Viennas at 89 db (don't know, haven't seen your impedance graph) due to the Maggies being very flat (impedance wise) with no dips demanding more current at a moments notice.
"My question is really if i need need anything more than say 50 watts per channel ? Why ?"
If it sounds like the music is straining, maybe more power would help. Could be the Denon is having a little trouble with the 4 ohm load and your speakers want more current than the Denon can supply easily.
Are you happy with the sound now? What you have going for you is the smaller size room and 75 db listening levels. If I was going to hazard a guess (and it's a hazard), If your speakers have some bad dips in their impedance combined with tough phase angles, you might get a more relaxed sound with an amp that can "easily" supply the current your speaker demands.
Go by how it sounds to "your" ears.
