For the Paramours the iron upgrade is hard to assess in isolation. The iron upgrade requires new 10uF Parafeed caps. So you are changing both the iron and capacitor at the output of the amp at the same time.
I instantly loved the iron upgrade in my Paramours. I put in PP Foil in Oil Obbligato caps, the black ones, at the same time.
Recently I was at Paully's house when he installed new all nickel output transformers and new plate chokes in his Paramounts. He did not put in a new Parafeed cap, just the iron. The improvement made me wish I had opted for all nickel. It was revelatory!
When I changed my output tubes a couple of years ago I went with the 2A3C from the ebay seller who delivers them for about $100-110 a pair. I really like them as well as some old RCA gray and black plates.
Several posters here have liked the JJ 2A3-40, try a search.