The upgrade to the 36.5 was a significant improvement over the 9.0. You gain a greater sense of space, weight, body, airiness, attack and decay of the music being played. Both are excellent preamps for the money, and Dan is the best in the business with respect to customer service. Tube rolling also helps to fine tune the sound to your liking, and some tubes will impart warmth, improve imaging, space and 3-dimensionality.
Regarding tubes, I loved a pair of Tung Sol 5687's with the 9.0. I managed to buy a pair of matched Kuhl Tube Tung Sol 5687's from Brenden, and those were a step up. I sold them not long after I upgraded to the 36.5. I used a metal base Philips GZ34 and Reflektor 6H30DP-DR's for the 36.5. Of course I have the Kuhl Tube 6H30 super tubes as well. Amazing.
IME, the best upgrades are to the components dealing with low level signals (DAC's, or other source, and preamps). While the amps and speakers are also important, they don't portray what they don't receive. Loss of information from the DAC will never be preamplified and so on.