Cheap speakers to go with Pass Labs Son of Zen amp

I have built a Pass Labs Son of Zen amplifier, and now need speakers to go with it. I am on a very low budget. I think the amp will end up costing under $70 after I upgrade the power supply. One of the main things I want is high efficiency since I only have a 15 watt amplifier. I have a pair of Yamaha NSA-636's that I am currently using, but their crossovers consist of 2 capacitors. Would it be better to build new crossovers for these speakers or build a new pair of speakers for under $150. The Yamaha speakers crossover is at 4.2khz and 13khz according to Yamaha, and the efficiency is 91dB. The speakers don't sound too bad, but I think I could improve on them without spending too much money. What drivers would you recommend if I choose to build speakers, and where could I find a design for a high quality crossover.