Ken,
Haven't tried the Siemens E188CC's in positions 2 and 4 but currently am trying some Russian made Voskhod 6n23p grey plates from 1975. There are many variations of this tube but these are reputed to be the best. They are relatively inexpensive especially when compared to a CCa but comparable or even better in some aspects IMO.
Send me a PM and I can provide the email of my source.
Tom