i would say 5-6k might be a minimum... as usual it is all about where one sets the bar for ’good sounding’
This is very useful, indeed. After all, it's likely a very good sounding speaker that's *not* sensitive can be had for $2k or less and then the money which would have been necessary for a really good sounding speaker could be apportioned back to the amp. In other words, if $5-6k for a really good sensitive speaker is needed in order to save on tube amps, the outcome here seems to be that spending on amps is fine. It becomes more about how the sensitive speaker w/ lower power tube stuff sounds, as a combo, compared to a less sensitive one with a more powerful amp.