300B SET or EL34 SET

IME, a true triode tube like the 300B will sound better that an EL34 tube  "wired" in triode.  Low level listening ability is more to do with proper speaker matching than tube type.

But if we just look at the characteristics of the tube itself; which tube has most punch in the bass, or best bass?
What I understand from your answers the hights also goes to 300B.

But if we just look at the characteristics of the tube itself; which tube has most punch in the bass, or best bass?
What I understand from your answers the hights also goes to 300B.

The EL34 will produce more power which could translate into more bass when driving a less efficient speaker.  A 300B amplifier takes more care in matching with the appropriate speaker. 

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.