My first thought when I read your post was "Wow, this is one of those rare, big rooms that will let the M60s shine." Hopefully you have a lot of freedom in that room to experiment with soundstage width and depth which results from how wide you position the speakers and how far out you can get them from the front wall. 14 feet apart is a massive width compared to most on here. If you can position their backs 3 feet away from the front wall, you'll enjoy a deep and wide soundstage.

At 20 feet back, when 1 Watt is fed into the M60s, they'll put out 70dB in sound pressure level. 70dB is a "quiet flute in the background" territory. To get to 24dB above that quiet level, you would need 256 peak Watts per side.

When listening to say Fleetwood Mac, the nominal sound pressure level may be about 80dB for moderate listening. That level requires about 10 Watts at 20 feet. When Mick smacks the drums, that's a 12dB rise in sound pressure demand - maybe more depending on how well-mastered that production is. To get from 80dB and 10W to 92dB, you need 150 additional peak Watts for a total of 160W peak per channel.

You can see then how some may see your space as both a blessing given your speaker placement options and a curse given the power demands. My advice is not to buy any additional components yet. Experiment with placing your speakers using your existing components. You may end up sitting closer than 20 feet and that will lower the power demand.

As for shrill highs, you may not experience it in your room. Avoiding toeing in the speakers can help with that.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated