I was going to wait to see what people posted because this is not my specific area of expertise. The Joseph Audio speakers are typical sensitivity for audiophile type bookshelves (86db / 2.83V). This means you really need a good amount of power to drive them to any sort of decent volume level (i.e. 50-100 watts).
For 8 watt SET tube amps, you really need to look for high efficiency speakers. It depends on your budget and what you want. These are usually horn-loaded speakers or single driver full range speakers. If you have a low budget, maybe try Klipsch Reference RP-160M bookshelves, which have a sensitivity of 96db:
http://www.klipsch.com/products/reference-premiere-bookshelf-speakers?model=rp-160m
Here are a few other options:
http://www.zuaudio.com/loudspeakers/
http://www.lowtherspeakers.com/lowther.html
https://omegaloudspeakers.com/collections/monitor-speakers
Lot’s of choices. I’m sure others have better recommendations. Some speakers can have efficiency up to 101db.