I wish I were smart enough to know how the filament voltage made it through the diode to power the filaments, but I'm not. ;-) It worked somehow.
"I don't know what it is that makes power tubes in the rectifier position have such a big bold sound."
Well, you're only getting a half-wave supply (one diode) as opposed to a full-wave supply (two diodes in the 5AR4). It would be noisier, but if there's anough filtering it would work, I supposed. Also reduced voltage at the output, which would change the sound. Mainly, though, I'm having trouble imagining why a manufacturer would suggest, or even allow, replacing a twin-diode rectifier with a single-plate power tube, unless it were a 6080 or similar twin-plate regulator tube. Another possibility is that the unit uses SS diodes and the tube rectifier is just a buffer to provide a slower turn-on. In that case I guess a power tube could work. But most power tubes aren't designed the handle the amount current that rectifiers can. So it seems like a risky thing to me. ;-)