How many watts per channel are enough?

If by low-to-moderate volumes you mean peaks up to say 90dB (perhaps pushing to the mid/high 90s), then you should be fine, with ample headroom -- just don't do something silly like set the seating 20'+ away.

I found 60 Watts/ch to be perfectly adequate on 93 dB/Watt speakers for "loud" levels supporting peaks over 105dB and seating about 8-10 feet away. A more moderate level buys you the luxury of some more distance, less Watts, and less efficient speakers -- probably still with more headroom to boot. Classical tends to be more demanding, with its huge dynamic range and typically high crest factor, but you should still be OK here. The only time I audibly clipped that 60 Watt amp was with a Sheffield direct disc recording of Firebird Suite, on an accidentally-too-loud volume level set during the REALLY quiet intro...then WHAM!!

In your case, you'd possibly start hitting trouble if you wanted to push peaks over 100dB.

To be sure, you'll run out of headroom real quick once you start pushing that
volume knob into loud territory. That's why they make efficient speakers and
powerful amps!

I'm a fan of efficient speakers for sure. Even those *honest* 93dB/Watt speakers
I had were a little on the low side for me. I found the step up to 95dB or 96dB to
be very worthwhile. Unfortunately, the efficiency ratings assigned by
manufacturers seems to be oft abused and "fudged". Speakers should be
measured anechoic, not just on a single frequency that happens to be a
significant peak, and (ideally) not at the expense of decent bass extension. In-
room efficiency ratings are...misleading.

I've seen a posting from a well known system/room optimizer who measured a
set of "famous maker" 97 dB/Watt speakers at 3dB LESS than a 96dB/Watt
speaker from a different manufacturer.