Philbag,
I have given a writing on ccomparing the Schmidt as delivered and using 6V6 tubes and EL-84's in another thread.
Here is a copy of it.
I have found that the THD yellowjackets work in a similar way compared to EL-34's as do the 6V6's work compared to a 6L6 tube.
You are going to have a slightly more British vibe to the tone, similar to an EL-34 but at approximately 18 watts (9 watts per tube). It might be just the way I look at
the EL-84 tubes, but I see them as smaller, lower wattage EL-34's. If you use a Schmidt that has EL-34's in it, as I did upon delivery, and then replace those tubes with THD Yellowjackets and EL-84's, the amp should begin to saturate earlier, but the volume difference should not be that noticable, until you really crank up the volumes. Then you will feel how the amp has less thrust in the volume for louder work. The mids will stay with the amp more like an EL-34 affects the amp's tone. The top end will have a bit of "chime" to it and offer a slightly more Voxy tone.
In the same regard, I hear and see 6V6's as being a weaker version of a 6L6. I replaced the EL-34's in my Schmidt to hear the difference between them and 6V6's. I got the earlier saturation as described with the EL-84's, and a slightly more even midrange tone and overall tone which a 6L6 would give up. The tone was not brighter or chimeier, necessarily then the EL-34's were, nor scooped and more Fendery sounding, except at lower volume levels in the first channel when set with a lower midrange. With 6V6's, I heard a slightly more balanced tone, midrange-speaking with the EQ all set at 12 noon as from the previous EL-34 tubes. The highs were balanced and the lows were also balanced with the mids. This was easily adjusted back to a more EL-34 type tone by boosting the mids just a hair to approximately 1:30 o'clock. The chime effect was not as noticable, but the upper treble has a slight "gliss" to it as would be given up by a 6L6 in a clean setting. Not a lot, but maybe a tad.
My preference between using EL-84's and 6V6's, is with the 6V6 in a Schmidt. The sound difference is just easier to work with for my needs and the 6V6 seems to like being pushed , but still offers a balanced EQ tone naturally while having a sweet saturated sound.
Of course this is just my humble opinion. Others here may hear something I do not in a comparison. When you are as old and deaf as I am however, what else would anyone expect?!
I was using an XOX Handle for all the tests.