I'm a very recent Havana owner, and last week, based on the recommendations in this thread, I picked up a WE JW 2c51 tube (1949 date code) and a Halide Bridge SPDIF converter. I've listened and made some interesting observations. First, the WE tube, compared to the stock GE, is richer, bassier and more realistic in almost every way, bit it's also brighter, adding more of a strident edge to recordings. (The Tung-Sol is brighter still.) And then, more surprisingly, the Bridge, with its asynchronous protocol, made the Havana sound more detailed, propulsive and pristine, but at the expense of the rich, gooey tone I've been getting with a direct USB connection using a Larry Moore (double-stranded) USB cable. In comparison, it made the recordings sound more digital—thinner, tilted up, and lighter. Not at all what I expected.
I realize that this is a question of synergy, and that my speakers (Tannoy Ardens) are fairly unforgiving of brightness and treble edge. I love the sound of the Havana, but I'm trying to banish the last bit hardness in the highs. So far, the WE tube and the Bridge seem to be busts in my application. I've experimented with polarity, switching USB ports on my MacBook Pro, and power cords (all create audible changes on the Havana). I have some Russian 6N3P-DRs on order. Can anyone recommend any other ways to relax the highs on the Havana? I'd love to make the Havana work in my system, and I'm deeply grateful for any suggestions.
Try the JKSPDIF Mk3 USB-SPDIF converter. It has an easy, relaxed sound, yet detailed, transparent, and natural. I much preferred it to the Audiophilleo, which I found hard in the upper mids/lower treble. Actually I will have one to sell in a few weeks if you're interested.
I don't think I've ever heard this DAC described as bright. What amp/pre are you using?