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

I would say PP amp do sound great. It really depending on the design to get the very best out from it. The most common problem of the manufacturers are trying to get maximum power from the amp; more the better. However, it pushes the power tubes working in the non-linear Class AB region. Even though the THD figure looks good, it doesn't sound right at all. 

My friend built a PP amp with 807 and I built one with KT66, EL34, or 6A3. The output power is less than 8 watts because we set the B+ to around 320Vdc and idle current of the power tube at 60mA. Also, both driver and power stages are using CCS at the common cathode to form the full differential/balance stages. 

We are so surprised how good our amps sounds that surpassed the single ended amps that we ever built.

Johnny

I noticed that both KT150 and KT170 are very linear in triode mode like the 300B and 845 DHT tubes. 

I expect they are the excellent performance if they can be used in small/medium power class A environment.