Have no experience but would be curious if anyone had success using 5751s. Just a thought. There are a few really good current 12AX7s. I like current production Tung Sols but not better than my Sylvania triple Mica Blackplates from the 50s. I just don't know how good a 5751 would sound in that circuit.
FWIW, Mullard 12AX7 reissues made a huge improvement in my C220. McIntosh has told me that, other than the phono section and a few features, the C220 and C2300 are the same.
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