could it be 6H23P-EB?Yes, it could. The 6H23 is an exact replacement for the 6DJ8/6922/7308 family of tubes.
You have lots of choices for replacement tubes.
http://www.upscaleaudio.com/vacuum-tubes/preamp-tubes/6922-6dj8-7308-pcc88/
Here's a good read about tubes and the SF Line 1 Preamp:
https://forum.audiogon.com/discussions/nos-tubes-for-sonic-frontiers-line-1