I thought so too, took some measurements at a potential operating point, I found gm and linearity were better in the EL34 than say, parallel 6BX7 at high voltage, 440V B+. Shouldn't have a problem with the 60mA grid current at peak swing, I have a pair of NOS EL34 burning a hole in my pocket too.
With a similar feedback circuit to the 801A design, the challenge is the gain. For a 3K load to get 15W output with a 3:1 damping ratio with the HK54, will need something like 25dB of feedback. I grabbed a couple of high-gain pentodes to try out, 6EW6 and 6AH6, the 6EW6 being the higher of the two. With a 5-pack of NOS 6EW6, what I found was a high degree of variation in gm / the curves from sample to sample, even from the same exact batch, BUT they can be made to be matched pretty well by adjusting the screen voltage, so I would need an adjustable screen supply to match the feedback in both channels.
So we'll see, it's going to take some trial and error.
