I have paired WP 5.1 with SF Power 3 before. Despite the power rating, SF Power 3 was not able to drive the WP 5.1 without clipping. WP6 might be an easier load than 5.1, but I doubt it is very different. But if you want to go the tube route, I will suggest SF Power 2 over ARC VT-100 II if you want to stay within your budget.
Tarsando is correct about WP 6 is why the sound lacks warmth. If your goal is to have warmer sound, start with new speaker search.
Tarsando is correct about WP 6 is why the sound lacks warmth. If your goal is to have warmer sound, start with new speaker search.