EL34 vs. KT66 output tubes

For Push and Pull tube amps,
I always prefer 7027, KT66, 7591 and EL84 over EL34.
The vocal always sound richer over all than EL34.
The bass on EL34 is not neccessary better. It really depends on which brand/type of the tube and the amp you installed them on.
Always prefer the NOS tubes over the modern Chinese and Russians.

"Most people, me among them, prefer the EL34 over the KT66 (and 6L6/5881)"
Trelja, I wouldn't say most.
9 out 10 audiophiles I know around NYC prefer the KT66 ( none chinese nor Russian ) over the EL34 ( all types )
Amp has a lot more to do with the tube.
Maybe most of the EL34 amps I've heard are less musical to our ears than the KT66/7027/6L6GC type of amps.
The only exception to our ears is the Vintage Marantz 9 PP amp from the 60s. That's the only EL34 amps we really like. Powerful yet musical ( except for the price tag.)
However, we did easily find many other KT66/7027/6L6GC and EL84 amps ( use it on higher efficiency speakers ) that sounded much better than the rest of the EL34 amps.

One should focus on the amp design and not the tube choice.

For Push and Pull application,
I got great results replacing KT66s with NOS 7027A for my application.
The 6L6GCs lacks control in lower frequency when compared to 7027A. Some might like the rounder and fuzzier sound of the famous black plate 6L6GCs.
The Russian and Chinese KT66s sounded even worst than the NOS 6L6GCs. A bit harsh which cause long term listening fatigue.
I don't really hear much difference between the RCA/GE/Sylvania tall/short bottle 7027As. Some of my buddies thought the RCA/GE sounded slightly warmer then Sylvania but we can't tell the difference during blind test.

Trelja,
FYI, I should clarify that I'm using only vintage gears because I prefer the sound from vintage gears much more than the modern gear.
I do agree that the modern EL34 amps sold a lot more than the modern 6L6 variants because no manufacture makes any good modern 6L6 amp these days but it does not imply that 7027/KT66/6L6GC are not as competent.

Also, not all 6L6 tubes are equal. 7027/KT66/6L6GCs are the high output tubes in the 6L6 family. They can produce same amount of power as the EL34 type.

The 6L6 variants were used for many guitar and organ amps.
There are abundant supplies of NOS 6L6 tubes. I'm not sure why manufactures today didn't pursue making the KT66 amps these days. Profit margin or it is hard to design a good KT66 amp these days since there werern't many clone circuits and good transformer available?

As far as EL34 vs KT66 tube goes for the modern home audio equipment, the modern EL34 has much higher demand and longer history than the modern KT66 due to hard to find NOS EL34 mullards and Telefunkens. Therefore, the Eastern European EL34 tubes has come a long way to make something decent when compared to its KT66 line.

I would not buy any modern KT66 amps in the future either.
I would buy the vintage PP KT66 amps.

Our friends here in NY definitely prefer the KT66/7027/6L6GC and EL84 over EL34 because the mids sounded more full bodied. Not sure if that was the reason why they use these tubes for guitar amps. They seemed to match our speakers much better than EL34s.
The EL34 might be more balanced for rock music but not the string instrument, vocal, saxaphone and drums just don't have the authority and tonal quality we like ( again, except for Marantz 9 mono blocks from the 60s.)

Again, just my opinion. I couldn't stress enough about amp design.