Getting 60 Watts out of a pair of 6L6's means pushing them hard. A pair of 7027A's would drop right in, and should be bulletproof in your amp. My 1977 Ampeg VT-40, (measured 70 Watts rms @ 5% distortion), is still running strong on the original tubes! The 7027A is readily available, and since they aren't as well known (popular) as the 6L6, they actually cost less.