300B SET or EL34 SET

6C33c based SETs are really much better than a 300B or EL34 .The typical 300B SET will produce only about 6 watts whereas a 6C33C will produce more like 15 watts and provide around twice as much current.There are very few speakers that can be driven properly by 6 watts .Especially when the amp has no negative feedback and high output impedance which is typical of SETs.That combination is a recipe for a very uneven frequency response.Hence the reputation of SETs for boomy and loose bass.I agree the 300Bs have a beautiful midrange .Which is why I use a 300B preamp-a Supratek Cabernet 300B.That way you get the good things of the 300B without the bad things.

I would tend to agree that for a more linear presentation, the 6C33 has serious advantages. The best solution if power is a concern, is to move towards a ultralinear or push-pull KT-88 6550 amp. You do sacrifice some midrange intimacy but most else is better. I think that the belief that any amp under 6 watts per channel or so can drive any speaker properly is mistaken. Again in all designs the transformers are critical. 

i honest think it’s more of the designer; how it was built and not the tubes itself.   Comparing apples to apples; 300b will for sure sound better in triode compare to el34 but if you are looking for power and low efficiency speakers than el34 would be a better chose.   Both are great tubes; it has it’s own strengths  and weakness.  

I agree with many comments here but I will say after running 300b's for a few years and having to retubing my amp I wish I gone el84 lol.  

I have very good transformers in my amp hashimoto's and do not feel I'm missing anything( over other tube verities) in the bottom end or top end, I have efficient speakers though 94db and 99db.

 I ran the 300b's on those 94db speakers for a couple years made great music but they were limited to lower listening levels so IMO your 89db speaker wont be a great match, doable but not satisfying IMO. 

also 300b's are getting very pricy compared to el34's I spent $800 this summer on 2 new gold lions, and another $700 on small signal tubes (2x12ax's and 2x6sn's). keep that in mind every couple years buying 300b's gets expensive fast, specially if your pushing them with low efficiency speaker they will go faster. 

@glennewdick  The 300B tube was designed for extremely low maintenance and a very long life.  As an example, WE300B are spec'd for 35-40K hours, therefore, making their cost of ownership quite inexpensive when you amortize the initial cost over its expected lifespan.  I know many individuals who have ran the same 300B for years, and the tubes still sound and test like new.