How good is to use integrated amp as preamp with the preout feature

Your exposure most probably has a decent preamp section and a lackluster power amp section. The accuphase could be an upgrade.

When its power amp section is not in use, i.e., leeching off the power supply for the preamp (assuming they share the power supply), the preamp will also start to sound better.

You can put the streamer, dac and preamp in the same box. Guys like Borresen will tell you that it is in fact a better idea to do so. In many cases, having a seperate everything (streamer, dac, preamp, power amp strung along with unnecessary cables) will sound lousier in the end. The power amp is essentially the only thing that should be separate.

Given the Accuphase  has gain settings you can find a level of gain that matches well with the preamp stage of your integrated. I also believe the Accuphase should prove to be an upgrade.  Much will depend on how the Accuphase mates with your speakers sonically to provide the presentation you desire.  What speakers do you have?

805D3 and Rel S510