Maybe Leaving SET for Solid State because I want bass

If this amps output power at 20Hz is one watt then you could assume it's a one watt amp and 20Hz is the bottom of it's power bandwidth.

@faustuss  That would be a lot (most) of SET products!

Most SETs have their power rating at a point where you can see the sine wave on an oscilloscope clearing being distorted. That’s a lot more than just 1% which is an arbitrary value. So I would not fault an SET that could make the same power at 20Hz as it can at 1KHz, but the only SETs that can do that are either parafeed or use the Berning ZOTL output circuit, which uses an air core output transformer. 

Since its the bass where most of musical energy exists, IMO its important for the amplifier to be able to make full power in that region. Our OTLs can make full power to 2Hz, which nicely solves the usual increase in distortion you see with almost any transformer coupled tube amp. 

These days though there are class D amps that are just as smooth in teh mids and highs as the best tube amps and don’t have any bandwidth issues. But some people really can’t get their head around that so here we are :)

Cut off the demand & effort of those lower frequencies from your single ended triode amplifier. Let the SET focus on what it does best. 

You could try something like the analog Sublime Acoustic (no affiliation) as a crossover or low pass filter...running dual subwoofers. Or any other type device out there that can cut off the lower frequencies from your amp without degrading the signal.

You could go on the journey of building your own sub or subs to specifically match your situation. Use a minidsp type device or Dirac just for the sub work & be blown away by the deep linear bass in phase with your mids & highs. 

Trying out & buying a new amplifier is certainly exciting. Also very rewarding if you are able to achieve what you are looking for without having to add subs..& having to put in the effort of dialing them in. 

@dmdalton 

You. Mentioned buying AS class D amps.

Do that!

While not quite as “magical” as a great SET, they have superb bass and overall surpass all but very expensive amps. I loved mine and only replaced them with a $45K Dartzeel integrated. I don’t think you can find any better amp for anywhere near their price.

Ralph has frequently said that his Class D are equal to any of his great tube amps.

I think they are better.

You may want to try the Pass XA-25….I have the XA-25 and the Sit-4, neither sound lean at all the XA25 gives me a tad more bass with my Volti Lucera speakers but I prefer the Sit-4 …the bass is very close…

@atmasphere 

"That would be a lot (most) of SET products!"

Probably all.

"That’s a lot more than just 1% which is an arbitrary value."

I wouldn't call it arbitrary, It's a standard. What is arbitrary is how tube amplifier manufacturers publish specifications which provide no valuable information to the consumer to compare one manufacture's product to another. They don't follow FCC guidelines but seem to get a pass because when they were initially instituted in 1974 the market had almost completely moved over to solid state and as long as a manufacturer used the RMS method of publishing their specs (solid state) regardless of what bandwidth they specified would be in compliance with the rule. Some continued to publish completely erroneous power output, distortion and noise numbers while others strictly adhered to the rule.

Interesting the review of your M-60 mono blocks by Charles Hansen from Glass Audio in 1998 confirmed, even relaxing the tolerance to 3% as he states - "The generally accepted practice for OTL and single-ended (SE) tube amps is to use 3% THD+N as the clipping point. This occurred at 23W into 8Ω and only 9.5W into 4Ω.
Maximum output power into 8Ω was 65W (5.6% THD+N) when the B+ fuse blew. The fuse blew again during the 4 ΩΩ test at 44W (8.8% THD+N). The AC line voltage was 118V AC, rather than the 120V AC at which the amplifier is rated, for 60W (8Ω) and 45W (4Ω)."

Relaxing the criteria to 3% he could not confirm your power output specification @ 1Khz. Further he had to relax the parameters to 5.36% at 8 ohms and 8.8% at 4 ohms @1Khz to achieve your rated power of 60 watts and 45 watts respectively which leads me to conclude that from 20Hz to 20Khz using 1% as a threshold your amps like any other tube amp on the market regardless of topology can only produce a small handful of watts at best. Let me qualify that saying that there are amps that use two or more parallel A-B pairs to be able to generate a few tens of watts at best with huge amounts of AC current draw and corresponding amounts of heat. You also have to consider the reliability issues with keeping all those tubes within several percent of each tube's bias.  

With your amps you have DC offset to deal with which is equally injurious to speaker voice coils as oscillation is and they use lots of tubes together in an effort to lower output impedance which are not common in the general market, that run hot and probably has a long-term effect on reliability.

Let me say any tube amp connected to an appropriately efficient speaker with a favorable EDPR and operated within it's actual power envelope (a few watts) will have a wide bandwidth, low distortion and a clean, fast and convincing presentation.

Interesting though I wasn't able to uncover any objective test bench evaluations of your OTL amps published since 1998. Only the subjective ones from publications that don't routinely do them.