I love singled ended 300B for its musicality, tone, refinement, basically that mid range magic. However 6-8 watt 300B in single ended configuration usually lacks low and high frequency extension. Is a well executed parallel single ended 300B amp putting out 18-20 watts the best of both world with better bass and yet retaining that mid range magic?
I have to suspect this is speaker matching related rather than a case of lack of power availability. I've heard enough SET 'single' tube 300bs to know that these frequency extremes are present(with a properly chosen speaker). Depending on the particular speaker in question(load/characteristics) a PSET could certainly be the better choice.You really must view the amplifier and speaker as one entity (inseperable pairing).There are listeners who obtain full frequency sound with 2 watt 45 SET amps if using an appropriate speaker.
I agree with Charles. Also as he implied, not all 300b SET amplifiers are the same; and not all 300b tubes are the same.
Edoit, Do you have speakers in mind?
I auditioned trenner and friedl speakers with 93 db and 8 ohm with verdier 300B. Bass was there but 845 mono blocks driving same speakers gave better low frequency extension. But I like 300B for the magic it does to midrange. That brings me to the question, will a parallel 300B have the best of both worlds using the same speakers?
I like both 845 and 300b SET amplifiers, I do understand your attraction to the 300b attributes. A PSET 300b could be an ideal compromise with this speaker.Some 300b SETs with identical watts/power ratings will still vary significantly in bass and HF performance due to design, power supply and output transformer quality.I like your taste in speakers by the way. Generally speaking the 845 will provide very good bass and drive. The 300b perhaps a bit more nuanced and intimate(there's much overlap between these two tubes in reality due to implementation).
Charles mentions power supply and output transformers. Try hearing a SET 300b amp with a beefy power supply like a Border Patrol SET 300b amp. You might find this amp gives you the bass control that you seek. If your like me, once you hear that SET mid range magic, you'll have a hard time living with anything else. A SET amp with a well implemented power supply may be what your looking for.
If you do go the PSE route, Audio Note has two kits amps that are PSE. I haven't heard the PSE kits, but I have heard the kit 1 in my home and it makes music. I have heard two 300b PSE amps but have never A/B'd them with SET amps. Folks who have heard both say they give up just a little bit in that SET immediacy.
I have designed both SET and PSET and still make both. My experince, as mentioned above is that a well designed SET should not compromize dynamic authority. Less power should mean less bass.
PSET amps done right can provide the best of both worlds. You are still within the single ended topology but simply addition another tube in the mix for more output power.
as mentioned, the power supply is every bit as importnant as the circuit design. A great circuit is useless with a proper power supply just like a great power supply is useless powering an inferior circuit. You need BOTH.....
Missed a word above......"less power should NOT mean less bass".
Agreed that power supply is critical, after all the power supply is what you are listening to, the rest of the amp is just there to modulate the power supply to make a higher-power version of the input signal.

For example, my 833C SET amps have 400Joules of energy storage per channel in the output stage, and they are NOT limited at either frequency extreme. Yello shakes the walls and floor quite nicely, actually better than my previous 500wpc class D amp!