The WARMTH effect will be a very subtle difference to the sound. However, here are a few suggestions to get the most out of it.
The meter for the 'warmth' effect is effected by the signal of the 'mic channel', 'instrument channel', or both, at the point in the signal path after the input/ preamp trim controls, tone controls, etc., but just before the master volume controls. The 'warmth meter' shows how hard the 'warmth' drive effect section is being driven, so the more gain you add at the preamp and tone-control stages, and the higher you have the 'warmth' dial turned up, the more chance there will be of seeing the 'warmth meter' moving.
For example, to set the input level, adjust the input trim GAIN control while strumming your instrument; keep turning up the GAIN knob on the channel your instrument is plugged into until you see the red preamp CLIP light come on, and then back off gradually until you find the point just before where the CLIP light would light up - the goal should be to set your input preamp gain as high as you can, just short of clipping. Also make sure that you are not turning down too great a majority of the (bass/middle/treble/attack) tone controls below '5', or you will be reducing the overall signal level prior to the 'warmth effect' stage. Then try turning up the 'warmth' knob and you should be able to see some meter activity at higher settings and during louder passages. If you had the input GAIN knob set to a lower level before you will likely need to turn down the MASTER output volume control to compensate for the input levels being set 'hotter'.
ACX1000 - Use of the WARMTH control (tube sound)
As the manual states, the best way to get full use of the tube emulation effect is to have a good signal in whatever channels are in use. The CLIP LED needs to flicker during the loudest passages of signal.
If the guitar signal doesn't register any indication on the CLIP LED or the WARMTH VU meter, it may be necessary to put a new battery in the guitar preamp, or consider installing a preamp in the guitar or acquiring an external preamp.