Near the bottom corner of each side of the 6UY there are standard speaker cable connections... Meaning each connection has a red (positive) and a black (negative) slot which accepts bare speaker wire. (no banana clips, etc.)
The 6UY only has an 8 watt output so if you're searching for some sort of bookshelf speaker, make sure that they are very efficient. I spent a lot of time looking into that as well and wound up just going with Panasonic made speaker for it (TY-SP42P5WK).