I am currently using my Wadia 27ix/270 combo directly into my monoblocks. I, like you, also have a world-class tube preamp but all the improvements you noted were evident to me also. It seems that, no matter how good the piece, nothing beats "nothing" in the signal path.
Sound without the Preamp ?
I thought I posted this before, but can't find it so here goes. I need some help here. I removed my tubed preamp and plugged the balanced interconnects directly from the 2 tube monoblocks directly to an Accuphase CD player with volume control. A weird thing happened. The sound got better, more transparent, deeper and tighter base. I find this indded strange because, this tube preamp is one of the industry's heavyweights, with a price to prove it too. Am I just "hearing things " ??