The KT88 is the stock tube for the 200S and will sound the best in it for bass. The JJ's are a great tube. I have a pair in my 200S right now.
For 6L6GC's, the 200S puts out a rather high voltage...usually around 560v or so. Thats really pushing the 6L6's and with a UL circuit on the amp too, its a bit much. I would say that if you put the KT88's in and rebias, it should work just fine for bass as long as they didn't mod the amp in other ways for guitar. You may want to check it and compare it to a schematic to make sure it is otherwise stock.