Maybe Leaving SET for Solid State because I want bass

@atmasphere 

Really helpful write-up.  I guess the question is, does this apply to the OP’s specific SET, which is an Ayon putting out 30 wpc, which is highly unusual, apparently due to is use of 62B tubes.  I also apparently had lower distortion than typical of SETs, at least according to this review:

https://www.theabsolutesound.com/articles/ayon-audio-crossfire-iii-pa-power-amplifier/#:~:text=What%20the%20Crossfire%20III%20PA,find%20counterbalances%20an%20SET's%20drawbacks.

@dhcod Part of this is the fault of a vintage preamp I put in the system for a few weeks. The tone controls showed just how much bass the speakers were capable of, even if the rest of the sound was less than ideal. 

I'd argue the opposite about the type of bass you are hearing right now as-is. Turning up onboard tone controls makes for a weird type of flabby bass too. I'd bet on a much higher quality preamp [with no tone controls] paired with two smaller / fast subs would give you a type and quality bass (and resulting upper and mid frequencies) you've never heard before in this system, imo. Just Sayin'. :) 

@mdalton As best I can make out the Ayon is no different in this regard, as the bandwidth has nothing to do with the power tube (other than lower power tubes tend to have smaller output transformers which can have wider bandwidth). 

I took a look at the article at the link you provided. 

They spec the amp to 8Hz but as best I can make out, that is not at full power. I've found no tests online to show which show its full power bandwidth. It would be quite interesting to find out at what power level and frequency the very low distortion spec mentioned in the article was measured. I did not find a distortion spec on their website. 

Since there is no mention of 'parafeed' or parallel feed on their site, my surmise is this amp uses a gapped output transformer. 

The transformer cans are round. That suggests toroidal transformers in use; the website says indeed the power transformers are toroidal. Toroidal output transformers saturate very easily with any DC that the only way to use them properly is with a parallel feed circuit. The round cans might also be cosmetic; there is no mention of toroids being used as output transformers. 

BTW there is an error on their website. It mentions in several placed a tube regulated power supply, but I'm pretty sure what they meant to say there was 'tube rectified power supply' since a 5U4 is a rectifier and not a regulator tube.  

@atmasphere 

Really interesting and helpful.  I’ve been tempted myself to go the SET route with even more efficient speakers than what I currently have, so my interest here is not merely prurient.  So when you say one should prevent bass getting in, I have a couple follow-up questions:  What level (hz) do you define bass?  And if a speaker is only rated down to Xhz, does that in effect mean that no (or little) bass below X hz will make its way to the SET?  In other words, do you need a crossover or filter to achieve the objective of limiting bass getting to the SET, or can the design parameters of the speaker itself (e.g., full range driver with no energy below 50 hz, supplemented by powered bass, so that SET only driving full range driver)?

@mdalton The minimum frequency varies depending on the output transformer of the individual amp.

I repaired an SET recently that made 12 Watts. When I tested power bandwidth, I found that it hit a wall at 90Hz. Below that its maximum output power without excess distortion dropped; at 20Hz it could only make 1 Watt.

The bigger the output transformer relative to the output power, the lower it likely can go. Lundhal is known for making some excellent output transformers but even their best is unable to allow a 300b to make full power at 40Hz- its down a few watts.

So I'm saying you should measure it to find out. But in general if you cut off at about 80Hz (and use a subwoofer) you'll hear a marked improvement in the sound of any SET (that isn't a parafeed) - more relaxed, greater authority and detail. All that is needed to do that is a capacitor in series with the input that is the right value. Such a part would not be expensive at all since it does not have to handle any Voltage.