I very recently switched over to running a matched quad of Tung Sol 7581A power tubes, which replaced a set of quad of Sovtek 6L6WXT+ tubes in my NS-40 amp.
Findings: Clearly noticeable improvements: better bass (tauter & punchier) / midrange (liquid w/clarity, those mids that tube amp owners love) / highs (beautifully detailed and effortless); more open, deeper, and precise soundstage; increased detail retrieval; improved dynamics / power / control. Simply all around better by a wide margin: the amp sings unrestrainedly and wonderfully, and I thought it sounded very good with the 6L6WXT+ tubes.
I understand and respect that some feel swapping power tubes may not help in their respective systems. For me, that was not the case, and my only regret is not purchasing the Tung Sol 7581A quad earlier. YMMV. Happy listening!