Thanks to you all for your "wise" words.
About tube rings, as I have already said in my last post, these Duende are perfectly equals to my tube rings (oops, to tell the truth, my tubes are black and are priced only 0,90 euros a pair). Really guys, you have been very kind to give me wise advice about my insanity to spend and not to ever rest (at least my wallet!) and I appreciate very much, but this time, why don't you listen to me? My Naim seller (as I always said,first of all is a friend so I believe in him) sells some kind of tubes rings as your Duende but he advised me against spend about 40 euros and took me to an Electrician's shop (I don't know how is the English name for a shop that sells all kinds of electric gadgets!). The black rings that I have bought there give the same and exact accomplishment of those that it sells for 40 euros (we have made a try in his shop), the only difference being that they cost 0,90 a pair instead of 40 euros!
I have put two rings on every one of my TS BGRP and I am very happy with them. So why you too don't try first the cheaper ones? if they are not working, you can always buy Duende rings in a second time.
In the past I have spent a lot of money "on magic things" (such as Shakti black stone, Quantum QX4, antiresonances cones, etc.) but to tell the truth, the only magic in these things is the (very high) price!