I use a low powered valve amp of 1.8wpc on a variety of (efficient) drivers. There is always a fear that SQ is subtly affected as the amp begins to run out of steam, not enough to be an annoyance, but enough to negate some of that extra money spent to squeeze out the last bit of SQ.

My simple tip is to wind the volume well past where you normally listen and retreat outside the room where distortion remains quite noticeable. The sound should remain sweet enough to invite you back in.