I agree with your comments regarding the Tenderfeet. In my review (which was not intended to be a review in the formal sense by the moderators apparently felt it was more appropriate in that category than in the general discussion), I mention the mini iso cups for my preamp but never really commented much about it. Although the mini iso cups did improve sound over not using anything at all, it was recommended that I try the Tenderfeet to see if they may more appropriate. Indeed they were. The improvements with the Tenderfeet under my preamp was a big a Wow as I got from the Cd player. A focused, firm musical sound and soundstage. Everything mentioned from Nonoise above is confirmed with my experience. Apparently, the Iso Cups work better with some applications and the Tenderfeet better with others. Much is system dependent and directed by personal tastes.
I not only give a thumbs up for the cups and the feet, but for all the Herbies products I have tried. I usually do not get this excited about audio products to post comments like these, but I have to say I've been so impressed by Steve (Herbie himself) tireless assistance, great price and great product that I cannot help myself but share the excitment. You also get a 90 day trial period so you can't go wrong with just giving Herbie products a try.
And, although I'm still in the audition with Hal-O tube dampers on the my amp, I am even more blown away by the improvements with the dampers than the cups and feet. After placing the dampers on the phase splitter and driver tubes of the amp, it wasn't just the clarity and cleaness of the sound that improved, but the overall tonality and musicality. With my amp, I had been noticing a small amount of coloration overriding the music on everything I played. With the dampers on, the coloration vanished. To be honest, I still need to do some more critical listening to verify my subjective conclusions, and the results with the dampers were not so dramatic when used on my preamp, but so far I was stunned by the improvements induced by the Hal-Os tube dampers. Highly recommended.