I though I had answered this post yesterday - something weird with the forum software I guess.
Anyhow, definitely worth experimenting. Anything from 2.7uF to 10uF is a pretty good range. It's hard to predict how it will interact with any specific speaker but in general the larger caps will provide more extended bass will less immunity to deep bass transient overloads. With the 604s and a 300B you are not likely to get into overload trouble :^) Try something around 6uF for a start. I always suggest if you are experimenting to use inexpensive caps, possibly paralleling to get various values (6uF parallel with 3.3uF is 9.3uF for example), until you find the value that gives the best bass - then spend for a good cap of the right value. Be sure to let the cps break in before making judgments though - all caps usually sound pretty thin to me until they have settled down.
