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. 

it's purely a matter of design

@inna That's the answer.

If you want to hear a really musical KT88 based amp, the Harmon Kardon Citation 2 (properly refurbished) is a good place to start.

It makes a difference too if the amp is pentode or Ultra-Linear and also what class of operation its set up for. You can't get away from the simple fact that the larger the output transformer is, the more critical it becomes and the harder for it to make bandwidth. IMO this latter issue might be the most important.

Ralph, I see.

I have an EL34 headphone amp that sounds quite good.  Among pentode/tetrode tubes, my favorites are very low powered tubes.  I run 349 output tubes in pushpull that deliver about 4-5 glorious watts per channel.  This is my favorites tube aside from the truly crazy rare 252 tube.  My next favorite is the 350B (a variant of the 6L6 tube), followed by the 6L6 or KT66, and then EL84 (a tube often used in less expensive amps, but, I’ve never heard an EL84 amp I didn’t like).  
 

Of triode tubes, either pushpull or single ended, I particularly like the 45 and 2a3.