So do the 421a tubes have the same overall sound as 5998 tubes? My take on the 5998 tubes was power... I personally find them to offer a v shaped presentation, with an emphasis on bass and treble. I found the (cheap RCA) 6as7g's to be more balanced sounding, but less intimate/refined.
I'd suspect that from their reputation, the GEC 6as7g's would be neutral and detailed, sort of like your CCa tubes. I also haven't heard for sure whether the 421A's are the same as 5998's or not.