Using an amplifier with variable gain

I have an AudioSource Amp Two (80 wpc) that has variable output. I've had it for the last 5 or 6 years. Anyway, I've always put the gain at maximum. So, I control the volume using my preamp, but I can't turn the volume knob much past 2. Today, I was thinking about it, and I decided to turn down the gain on the amplifier to about 50%, and then turn up the volume on the preamp by a corresponding amount. My question is which situation should give the best result, in terms of sound quality? It' not easy for me to tell by ear, as I'm sure most of you would recommend to just listen. Is there any theoretical reason why one setup is better than other? Any advice is much appreciated.