what brand of tubes to buy?

Have a parasound hca1200 amp,a friend has put a darad 2007 special edition SL2000a tube pre in my hands to play with. It came with 2 sets of tubes (china 12at7) which sound great on hard rock( base pressures that make my eyes pulsate at just above normal listening levels LOL) and a set os sylvanias that are beautiful for acoustic jazz and the like.I can see why people roll tubes but short of buying every brand and experimenting(and spending stupid money)how do I make choices on brands and what differences should I expect? Obviously new to tubes,but blown away by the improvement over ss pres.any advice would be greatly appreated!