What combinations of tubes have you found to synergize?
I'm a bit perplexed by your question. How well combinations of tubes synergize depends mostly upon the circuit/component you are using them in, as well as on the specific tubes you use, not to mention other components in the chain. Just because 6SL7's work well pushing 300B's in one amp in one system, does not mean that's going to be the case in all amps using those tubes...and the which 6SL7's, and which 300B's. The variables here are staggering. Rolling tubes within a specific component can render huge differences, or very subtle differences or none at all. One interesting detail I've found over the years is that the closer the component is to the source, the greater the potential for impact when rolling within that circuit (rolling tubes in a pre or in a tubed-output digital front-end tends to have a greater impact than rolling tubes in an amp). This, of course is a very broad generalization and there are certainly exceptions. All of this is highly subjective as well, as anything in this hobby. Glad you're having fun with it.