Michel, i think that WA22 is not a real OTL but have a small output transformer when the low impedence is swiched on (You should probably check with W.A.).
Also on the WA2 standard drivers gives more work to the power tubes than 6SN7 or ECC31 or 6N7G this is probably why on WA2 you get some distorded sound.
On Elise, If i do not listen too loud i do not have any distortion with 5998. Also if i have 2 6BL7GTA by channel and even with 6n23p.
When WA2 is not to loud everything is fine but i think it lacks power as for WA22 it is a OUTPUT TRANSFORMER COUPLED amp