Hi Anthony
Congratulations on the GB4000 - it is an amazing machine. With regard to "feeling" - typically, the body's ability to feel something happening starts to drop off around or before 4000Hz so even if you are using Audio mode on the GB4000, you won't feel the higher frequencies. The main indicator then is the red lights on the two machines.
I love Hanks suggestion of testing by the tongue but holy ****!I think I would be more inclined to wipe one arm with a damp cloth - hold a cylinder in that hand and then touch the other cylinder further up the arm and move it slowly down towards the cylinder in the hand until you feel something.
In my experience, if the GB doesn't have an effect then it is purely because we don't have the right frequencies - with the right frequencies in fact it is not possible for it (or any frequency machine really), not to have an effect.
I have not had anyone with diabetes or tinnitus to treat so can't comment there but I think it is important to state that as much as I am in love with the GB, I would not (and do not) rely on it to be a cure all - partly because of the frequency issue. So I CAN tell you that I cured a friend of tinnitus by a neck adjustment (I studied spinal manipulation as part of my martial art training) so it might be worth talking to a good chiropractor (if you can find one).
I would absolutely continue with treating the diabetes with the GB (maybe researching more frequencies) but other things that have proven to be very helpful include Alpha Lipoic Acid, Astaxanthin, Bitter melon (type 2 in particular), Chamomile tea (can lower blood sugar by 25%), Clove oil (2-3 drops in tea or coffee apparently can be amazing), Gymnema Sylvestre (also amazing), and food grade hydrogen peroxide (make sure it is food grade - apparently particularly good for type 2 diabetes).
A couple of references that might be worth pursuing include: "There is a Cure for Diabetes" by Dr Cousins, or check out the video Simply Raw: Reversing Diabetes in 30 Days" at www.rawfor30days.com. Might require you to go "raw" for a month (yikes) but both claim that diabetes can be reversed.
For the tinnitus - aside from a good chiro the only other thing I can suggest is acupuncture.
In acupuncture, it is believed that tinnitus can be linked to sinus, kidney, and/or liver issues so running the normalising frequencies for those organs and perhaps the sinus program might also be worth a try.
Hope that answers some of your questions and helps.
Rgds
Tom