Well, let's give it a try.
Auxiliary (Aux) feeds on each channel are ways of sending a extra feed of that channel for various forms of processing, including things like the reverb you want.
Most mixers have several Aux sends on each channel giving a choice of "Pre Fade" and "Post Fade". As the names suggest, Pre Fade gives you the signal prior to the fader so your Aux send is always at full level even if you pull the fader right down--this is useful for setting up things like monitor mixes totally separate from the main mix.
Post fade auxes are affected by the setting on the channel fader--and this is probably what you want for your reverb since you want the reverb to adjust as you change the channel fader.
So...pick a post fade aux on your mixer and turn up that knob on each channel you want sent to the reverb--you can do it with just one or, for example, all the vocal channels at once. Besides the aux send on each channel, there will probably be an aux send master over in the master section of your mixer (I've never used your Tascam). Cable the appropriate aux out socket on your mixer to the reverb in. Set up your gain structure so that, with the channel faders up at zero you're getting something around zero level into the reverb.
Then take the reverb output and feed it back into your mixer. If you're short of channels, there's probably an "aux return" or "effects return" but, if you have spare channels, I prefer to bring the reverb back into a standard mixer channel since it's easier to adjust with a fader than a small knob.
Set your reverb box to output "full wet" if this is an option--you'll be adjusting the wet/dry balance on the mixer.
Done this way, any time you play back something via the mixer, anything on a channel with the aux send turned up will go to the reverb unit--and, by adjusting the balance between the channel fader(s) sending and the one you're using for return, you can tweak the amount of reverb you're using.
Obviously you'll have to route the channels in the normal way to get the reverb into the main mix (or wherever you want it).
I know it sounds complicated, but once you start playing it should make sense.
