The preamp tubes may still be good. That said, it doesn't cost that much to replace them. NOS has the best quality and sound generally, and they usually last longer than modern stuff, but the price is often high. Also, buying new tubes tends to help these companies stay in business so they can keep making you new tubes, so it is a good idea to buy new for that reason, and there are many types that do sound quite good.
Good modern brands to consider in no particular order are:
JJ, Winged C, Tung-Sol reissue, Genelex reissue, Sovtek, EH, Valve Art, Shugaung, TAD, EI.
Not all of these brands make the tubes you need, and some of the tubes in your amp are available only as NOS. Good NOS brands are pretty much almost all of them in no particular order.
RCA, Tung-Sol, Genelex, Mullard, Telefunken, RFT, Valvo, Amperex, GE, Westinghouse, Tesla, Tungsram, the list goes on.
Greg
