Buddy, welcome. Glad to hear how you're now enjoying the more accurate sounds coming through your M60s. Yes, when you've been used to something else for a long time, even an objectively better speaker might not seem great on first impression.
I'll comment that if you wanted to stay with Denon and for about $300, the 1912 factory refurb for about $320 from Accessories4less here
would be much more up-to-date in its features. Although the 5700 has a higher maximum power rating, this shouldn't be significant in actual use.
Also, there are excellent quality CD or CD/DVD players available for about $100-$200, and this would seem to be more cost effective than simply buying a DAC.
Yes, classical music is something that can really be enjoyed on better quality equipment. You might take a look at some of the classical recommendations in the "Stunning" thread.