300B SET or EL34 SET

If speakers were properly matched to the amp, I don't think there is much of a difference in terms of bass performance between and EL34 and 300B.  Both will be a bit flabby compared to SS, but neither is much better than the other.  The 300B is the better sounding tube overall IMO. 

What matters most in terms of bass reproduction is the transformers.  Great transformers will deliver great (for tubes) bass.  Crappy transformers will deliver flabby bass even by tube standards.  Given the massive power draws, transformers for SETs are critical.   

Regarding those specific amps - the 300B delivers a shockingly specific 8.2w while the EL34 delivers 12w.  

It is no secret that 1W of power is adequate to deliver music at normal listening levels.  You need headroom to hit peaks in volumes because music is dynamic and SPLs vary from moment to moment. Average SPLs for quiet listening might be 75dB but you will find that volumes go up dramatically.  Get out an SPL meter and you will be surprised to find that peak volume's approach 100dB (especially at low frequencies) all the time.  

Assuming normal conditions, sitting 8' from your speakers and they are 2-4' from the wall, 8.2w will give you a max output of 96.4 dB.  That is really low.  12w will give you 98.1dB.  Normal listening will be no problem but you will find that bass will get very flabby with dynamic peaks in volume.  

For music, I like to target 100dB in peak output when matching an amp in terms of minimum power output.  This assumes the amp is well suited to dealing with the speaker impedance.  Those speakers really need a minimum 20w amp.  

If you are curious, this is a useful link where you can do quick SPL calculations.  

https://myhometheater.homestead.com/splcalculator.html

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.