I own a 390S, 336 amp and a 38S preamp and I run the 390S directly into the amp. This means I have to disconnect inputs when I play a record or listen to the tuner but I've found the sound to be slightly more transparent.
As Nuno mentions, give it a try and see if you can hear a difference.
As Nuno mentions, give it a try and see if you can hear a difference.