Friend
If the Yamamoto is a 45 based amp (1.5 to 2 watts output) it will drive the JBL K2S - but not with authority. The K2S is 8 Ohm, 400 watt max, 45Hz to 50Khz, and 94 db sensitivity. I currently use a homemade 71A amplifier at 1 watt max output on modified Klipsch 3.2 horn-loaded speakers (93db sensitivity). While I enjoy the gorgeous detail - I am hungry for some authority. Therefore, my next project may be a 300B homemade amplifier.
The VTL line of amplifiers will drive the K2S with authority, but when you move to pentode amplifiers - you loose some of that "realism" that triodes bring to the table.
Visit www.davidberning.com and check out the Siegfried 811 amplifier. The 811A triode is the best big triode that I have ever heard! Hope this helps.
Studio1