Preamp for tube amp and for VTL Deluxe 120 Mono

What type of a preamp should be used with VTL Deluxe 120 Mono? I am using a Nad320BEE as a preamp and although the volume knob is turned all the way up, there is only a small amount of sound coming from the speakers. I supposed I need a high-gain preamp for tube amp in general? Is an Adcom 750 be OK?
That should work OK.

1. Disconnect the speakers from your power amps and connect them to the NAD and put the jumpers back. Are you getting good volume?

2. Connect the speakers back up to your power amps, put in something very soft that builds in volume (like Bolero) and connect the CD player directly to the power amps. Be ready on the stop button as you have no volume control. Is that loud enough?

Can we eliminate some possibilities?