how much power

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.

Something else to consider is that the Denon receiver is rated for
a minimum impedance of 6 ohms, and the speakers are 4 ohms.
I’m not sure if this would be a problem with just a pair of speakers,
but running a full surround system, I think it could be a problem.

esp. with your reasonably efficient speakers, it's the quality, not the quantity of power they're fed. the weak point of most avrs (incl. denon) is the amp section--one of the most significant night-and-day improvements i've made was bypassing the amps in my denon avr and using an external amp to power my front speakers. even a modest 60w adcom amp sounded much better than the (purported) 130w amp section of my denon.

Tls49,
If the amp is rated for 6Ohms minimum, than it will not have enough current for 4 Ohm speaker. Not only the output power is jeopardised because of mismatched impedance and lack of current, but also the remaining power is jeopardised by heat.
If OP gets half of rated power to the speaker, consider that as a best case scenario where the worst is fried amp even on moderate 75dB volumes.

Conserning Onemug's statement in general it's true that 50W and 100W will have a slight difference in SPL but in this particular case it's a substantial difference.

So Roxito,
You can have amp as low as 50Wpc, but it should be rated stable for 4Ohm load or preferably lower.

You just need better amp, not necessarily more powerful but probably higher current with more sophisticated power supply. Vienna are good speakers and deserve good amplification.