To start off, here is a link to some very useful information.
All things being equal to someone else’s amp (voltage, bias, load) then a similar tube should sound similar in your amp. The load is your speaker, and every speaker responds differently to different frequencies, meaning the load changes at different frequencies depending on the speaker. Also just because the speakers nominal impedance says 8 ohms, they are never spot on. So now the load gets amplified through the output transformer between say 300 and 1000 times, depending on which tap you choose.
If you have read the link, then you know that for every tube you would need to set up a load line to see how the tube would respond to an input swing. This is where the plate voltage, screen voltage, and bias point come into play. Adjusting any of these, changes how your tube will sound.
Now all amplifiers were originally designed off of the tube charts a given company would put out. Over the years people just based their amps off of someone else’s, and now you have people wanting to run 400v across their 6k6’s. Now you can do that, but not without some other modifications to the circuit. You would still have to set up a load line to see how the tube would respond. The two stroke amp was originally designed as a parallel single ended amp for dual 6v6’s hence the name. Over the years the schematic has changed and it is now designed for a single 6L6/EL34 type tube. Even if you change the impedance switch, you are only changing the load that the tube sees, that does not mean the amp is suited for a different tube. The biggest problem is the plate and screen voltages. If you did want to try low power tubes with hopes of a good response, then you would have to bring these voltages down closer to 300v. The link I gave will help to find the right cathode resistor for a given bias voltage. Obviously that link is describing triode curves, but once you learn it, it is easy to figure out pentode curves as well, except with different screen voltages from the ones they recommend, then it gets a little more tricky.
If you would like to lower your B+ to try that out with different tubes here is a link to a possible solution.
I have not personally tried this method, but it should work fine. You could even put in a switch so you could change the B+ from 300v to 400v. The things I had mentioned in the earlier post were about the driver tubes. Once you figure out your load line, then you know what type of voltage swing you want. Some output tubes also like a little more driver current. This is where your choice of preamp/driver tube comes into play.
Like I said it gets troublesome talking too technically, and I have just barely scratched the surface. That being said, sometimes the sweet spot on tubes lies outside the recommended values, but not very likely.