The best source IMO are blueray discs. You will need to get a blueray player with RCA outs to connect to your preamp OR get a preamp with an HDMI connection.
I have been ripping a ton of concerts lately, sound fantastic.
Yes, to a NAS, if you want to create a library and a "concert vault" consider getting a Zidoo player that will have storage as well as play everything. This looks like a good choice:
Let’s ask it another way, where can you great concerts other than blueray. You can stream a lot of great content. The Zidoo has an android OS and you can get geat apps for concert streaming. They won’t have the dynamics of a blueray but thre are tons of content, they add new content all the time, and its cheap:
1) Qello Stingray
2) Plex (has various concert channels that are FREE)
3) Berlin Philharmonic
4) Tidal (streams videos of concerts)