I seem to have resolved the problem - fingers crossed.
I narrowed this down to a number of clean tones. When having either of them active and going into and then out of Tuner mode, there would be a huge multiplying effect on the reverb. Please see attached audio file - I go into and out of tuner mode about 20secs in. No cables for my direct recording so just a mobile phone recording :wub: .
.......OK, don't seem to be allowed to upload .aac files. Here is a link if mildly interested:
What has happened is that I noticed that the reverb effect in all of my clean presets had a Mix value of "VAX V" and was not adjustable. These tones are all based on the same original clean tone which must had the same setting, a clean tone I originally imported from a HDPOD Desktop. Please see the attached image file. I have no idea what VAX V means, however, when I reset the reverb (by changing from one reverb to another and back) and then adjusted my settings ('VAX V' - does not appear), I am now able to go into and out of Tuner mode without the reverb effect multiplying.
bigkid's post in Massive Reverb Effect Kicks In For No Apparent Reason! was marked as the answer