With the AMP1, the amount of REVERB mixed into the dry signal is controlled by the REVERB pot on top of the amp.

However, you may have noticed that the reverb component of AMP1 also changes depending on the volume setting of the preamp: if you turn down the Overdrive Master in the overdrive channels or the clean volume in the Clean Channel of AMP1, it decreases the reverb component too. Conversely, the more you crank up the Overdrive Master or Clean Volume, the more reverb you get.

Don't let this unsettle you, though! This is completely normal and is in the nature of things. As you can see in the AMP1 signal flow diagram‍, the reverb sits after the preamp and therefore after the two controls for Clean Volume and Overdrive Master. The more these two controls are turned up, the more signal level is sent into the reverb. And the higher the level of the reverb, the higher the proportion of effects produced.

You can check this phenomenon by turning Clean Volume / Overdrive Gain down almost to zero. Then you will find that the reverb is virtually inaudible even when the REVERB pot is turned all the way up.

If you want to achieve a higher reverb component even at low volumes, you have to turn up the Clean Volume or Overdrive Master accordingly and instead reduce the master volume of the AMP1 using the MASTER control.