what is the best speaker for a flea power amp

Not certain what you want as a speaker is a personal choice. Very important to have effecient speaker with fairly flat impedence curve for a SET. There is a nice list of effecient speakers and a nice short discussion of what is important at www.welbornelabs.com/recomendspeaks.htm

Also there are lots more tube people at audioasylum.com in the eff. speaker forum. Ask there too.

Check out www.melhuish.org for interesting ideas about single driver speakers for SETS.

Hope this helps.

Sincerely, I remain