I did even more reading in the thread, and found this:
MULLARD, GENALEX, BRIMAR, and other British made NOS: Like a warm British jacket of the finest tweed, these glorious tubes have an attractive sweet warmth in their midrange and lower regions. The top end is silky and pleasant, without being rolled-off. The best of these tubes retain a fine sense of "air" at the top, and the upper midrange is smooth and liquid. These tubes reproduce the human voice, especially female voices, with haunting realism. The 1970s Mullard made have an attractive sparkle at the top with the rich bass, and these tubes are usually priced less than the older types.
I don't know very much about any of those brand names, but that is almost exactly what I'm looking for in sound.
Can anyone give me a recommendation for something that sounds like that? It doesn't have to be one of those brands, but I imagine that is the first place I'd start looking. Unfortunately I don't have the money to buy this unit and then spend hundreds of dollars finding a tube that sounds roughly similar to the description above, so any input regarding such a tube would be appreciated.