I found the Delta15 is a good compromise of price and performance and with 52 ohms Zmax, it is good for tube amps too. Wayne's designs can handle a lot of power but because of their high efficiency, that's another reason they work well with low powered amps.

Ask Nelson what the output impedance of the Son of Zen is; I think it will be under 1 ohm. My Decware Zen is 0.8 ohm.

If you have the space I would go for the Theater Seven Pi.

Good luck!
Are you doing this DIY?

Jeff