Are EL34 based amps more musical than KT series based ? Or it's purely a matter of design?

in my experience:

in General, Absolutely.

Absolutely correct about 300B amps. So much is the power supply. In fact, it took an expensive, properly engineered 300B amp to open my ears to this tube as viable. Have never heard a good 300B amp for less than 15K. 

If midrange is not right, the rest doesn't matter.

Power cord can make quite a difference for dynamics and bass, by the way. I use older Purist Audio Dominus Ferox, appears to be a perfect match.

Agree with most posters here, EL34 easily my favorite push pull amp tube. Sweet, natural mids, highs may or may not be rolled, depends on amp, bass is biggest issue. Bass issue is lack of slam, can be too resonant in some situations. Power supply in amp/speaker choice key here, get this combo right and bass can be ok. Still any KT will beat it on slam, extension, problem with KT is never get that same sweetness, natural mids, highs can be pretty nice, mids are the issue for me with KT.  I also generally agree the lower power KT sweeter than higher power.

In the end, the EL34 and push pull didn't do it for me, transitioned to 845SET than 300B SET. 300B is a magical tube with the right speakers and properly engineered power supply in amp, with this no roll off in highs, no soft, overly resonant bass, some totally misguided on stereotyping 300B sound. The boutique 300B tubes absolutely necessary. Running 300B in parallel can be replacement for those running EL34 with speakers SET can't handle, some very nice amps in this category out there.

@yogiboy On my Quicksilver amps I can use both types. I prefer the sound of the KT88 compared to the EL34. It all depends on the speaker that you are using. I find that the KT88 sounds bigger and brighter than the EL34!

+1 yogiboy. 100%, so true. Was just sharing this on another thread about this, its a fact. Matching to your preferred sound is key, all speaker dependent for sure.  

I love the sound of EL 34 tubes, reminds me most of tube amps from the past.  A ripe romantic type sound but without any bass slam. 

mulveling, "ain't cheap" is an understatement, VAC's top and near top of the line separates are extremely expensive, but you say they are worth it.

I have no complaints regarding the dynamics and bass of the VAC Avatar SE EL34 based integrated. I suppose, it also depends a lot on particular signal tubes that you use. These 60 watt/ch in ultralinear mode sound quite powerful.

So, there is something inherently very musical in EL34/EL84/6L6 tubes.

Matching and preference will play a role which one you like.

I think it’s hard to make an EL34 not sound sweet. You have to really mess it up. The larger KT tubes will (by comparison) give you brute force, dynamics, impact - but it’s hard to keep all that while getting them to sound as "sweet" at the same time, as the smaller tubes which seem to do it so naturally (EL34, EL84, 6L6’s etc). I think VAC’s recent high-end lines do a GREAT job at that (plus next-level articulation and refinement), but takes some doing and ain’t cheap.

Vintage tubes amps are a lot of fun and a nice break from the "hifi" sounding amps because they’re so inherently musical. Part of that, I think, is the use of those tube types. But there are those limitations - power, and usually bass impact / dynamics.

That's another reason I like those QS.   Sometimes I'm looking for a different presentation and will re tube with a 6l6.    My last quad was 6l6,  was just missing the el34 tone a little

It is a matter of taste and specific implementation.  While there are some rough generalizations one can draw about the sound of particular tubes, I don’t see such generalization extending to the KT label.  I generally don’t like the KT88 tube, but I do like most amps employing the KT66 tube.  That tube sounds most like the 6L6, another tube I like a lot.  

That's probably why I kept my Quicksilver Mid Monos.    They ship with EL34 but can take any EL34 or 6L6 variant.    I've used KT 66,77,88 but I lean more toward the EL34 so I will put them in my system occasionally.

I have a 300b too and prefer that but sometimes I'll mix it up and put the QS in 

There's a bunch of variables involved.  It's a matter of preference, the brand and age of the tubes, the unique circuit it's playing in, synergy with the system, and a whole bunch of other stuff.....definitely not an absolute.  It's also one of things i love about using tubes....try a new tube, and discover a bunch of new subtle tonal characteristics. 

IMO purely a matter of preference. I have sets of both, but generally lean towards the EL34 sound… finesse over power 😉…

On my Quicksilver amps I can use both types. I prefer the sound of the KT88 compared to the EL34. It all depends on the speaker that you are using. I find that the KT88 sounds bigger and brighter than the EL34!

https://upgradehomedevices.com/kt88-vs-el34/#:~:text=Bass%20of%20KT88%20and%20EL34

I think that it might be a matter of taste.