I'm not sure...

The 7027 is like a big 6L6 in tone, where the 6550 is a bit different. I mean, the 7027 is similar to the KT77 and the 8417. A big beam tetrode sound like a big 6L6. The 6550 is also a beam tetrode, but kinda harder sounding than the 7027. Anyway, I'll look at what is available and discuss here.

I'm just pondering the mods right now. Looking at what sort of preamp mods can be done and thinking about how to get the cathode bias mod to work in both full and half volume modes. Ponderin'.

I may end up having to use relays to get some switching done.

"Tyranny, like hell, is not easily conquered… What we obtain too cheap, we esteem too lightly: it is dearness only that gives every thing its value. Heaven knows how to put a proper price upon its goods; and it would be strange indeed if so celestial an article as FREEDOM should not be highly rated."
-Thomas Paine