Re: What EL 34s do you like?
Is that Rivera an EL-34 model? You'd probably be fine with any of the tubes listed, but for that amp, I'd probably put in Tesla JJ EL-34's, since they have nice glassy headroom to counterbalance the inherently compressed sound of a Rivera.
The E-34L's are also good, but may add more volume than you'd want to add to a 120W amp. I'd probably use the same type of pre tubes that sound best in highgain amps, the GT12AX7C, since they have a very tight focused bottom end, which Riveras can benefit from. If it's all tubed up with that combination, you'll have a great sounding, but very loud tone krusher.