Krell Showcase 7.1 DACS

This may be a stupid question, but here goes...

1. What happens (in terms of sound quality) if I feed a Sonos Z80 or Squeezebox's digital out directly into the Krell Showcase 7.1 Pre/Pro's digital input ie use the Krell's internal DACS

Would it be better to use an external DAC in between the Sonos or Squeezeboz and the Krell?

2. Is the Slim Devices Transporter internal DAC better than the Krell internal DAC?

Thanks for your help. I appreciate it!