I did a LOT of reading, and the Tenergy Centura are one of the best, and reasonably priced. The reason I went with the (more expensive) Imedions is that they are 8 cells instead of the 7 as used in the tenergy, resulting in a nominal voltage of 9.6V rather than 8.4 per "9v" package. 7.2 is even more common (6 cells). Given the relatively deep bias of the quickie relative to the supply voltage, I didn't want to give up any voltage so went with the 8 cell packages. The resistive bias slides relative to the supply voltage stock so it's less of an issue since the stock design allows for some voltage deterioration, but if someone were to use a led cathode bias (which I have preferred in another simple plate-amp circuit), that low a voltage in the supply would mean the led bias on the cathode is relatively deep and would be better around 1v than the 1.57 of the preferred HLMP-6000 part (which has excellent low and stable impedance). 1.57 ought to do well (probably better than the stock resitive setup) at a higher voltage- less input headroom but should be improved linearity and current availability (which will drain batts faster, a non-issue).
We shall see! The stock kit is designed to be flexible WRT battery types, system connectivity, and battery discharge curves but if one optimizes for a higher, stable operating voltage (perhaps even adding 1-2 more 9V cells) at a modest bias (trading input headroom for linearity and current), the linearity should be improved particularly with the PJCCS in place.
That's what's great about an inexpensive high performer like this- customization to your specific wants/needs isn't something to be feared, worst case, you buy $1.50 worth of replacement resistors and throw 'em back in, if you couldn't cleanly remove the originals.
