This is just to inform people that own this amp that some reviews of this amp may honestly not understand the MHA200. I just purchased this amp for a recliner setup using a Fosi K7 as the preamp. My main setup which is an RME UCX II feeding a Topping E50 and L50 are my primary setup and Bottlehead SEX v3 with Speedball upgrade tube amp. I also have a portable Cayin C9 tube amp. Headphones: Audeze LCD-2 Fazer, ZMF Atriums, Dan Clark E3, HD6xx, AKG K52 and HE4xx.

This amp is NOT an OTL, it's a proprietary patented output biased transformer tube amp. This amp DOES NOT follow the 1/8 or 1/10 ohm on headphone ratio rule but the rule of: MATCH the ohms for maximum output or consistent output rule, because it's is a transformer coupled tube amp.

The difference is pretty drastic and key to understanding how this amp works and knowing it doesn't use a volume knob but a GAIN knob. A standard volume knob is at the end of everything, it simply lets more signal through. A gain knob (which the MHA200 is) is like on a guitar amp and CHANGES the way the amp will sound. According to a review by ASR, as you increase gain the percentage of the harmonic distortion to the fundamental goes down as shown by his measurements, in other words you hear more harmonics vs the fundamental as you increasse the gain.

1. 12'oclock you have a neutral signal, sounds like your traditional neutral sounding amp but you're going to need some serious preamp power 8 volts (more below).
2. 1'oclock position, now this is according to the paid "thus supposedly better" Chat GPT I used (so take it for a grain of salt), I uploaded all the graphs from ASR and asked it to compare it to standard McIntosh amps, it used measurements for the MC275 from stereophile (if anyone wants to confirm feel free to read its reference and let me know if it got it right LINK) is at the 1 oclock position as tested by someone on ASR. (This is nice as a standard 2 volts being fed in is powerful enough for most headphones and all the ones I listed above for me.)
3. 2'oclock position even more harmonic distortion compared to the fundamental. (aka marketing jargon lush/warmer/darker/more umph)

Now on to the even more deeper nuances of this amp, since as an output transformer coupled tube amp, the MHA200 gets you full power at all the different impedance levels, sorta. See this is where it get's complicated, and I'm going to talk about dynamic and planar headphones.

DYNAMIC HEADPHONES: Dynamic headphones resistance changes, similar to SPEAKERS, thus even when you select a tap, the voltage and current is going to shift a bit as resistance moves around. This is why we have the 1/8 or 1/10th rule that the amp should be 8 to 10 times higher input resistance to output resistance, for a solid state amp or traditional otl amp as this ratio and more current fixes this. Doesn't apply here. Here McIntosh is trying to match ohms to your headphones to maximize power, similar to how their speaker amps work. When you are using a lower impedeance headphone and using a higher tap ie you use 32 ohm headphones and using the taps at 600 ohms, you're going to get more harmonic distortion, thus the more you go up up from the recommended tap setting the darker or more harmonics distortion you're likely going to get (which people want to do because their preamp more later down this post). Also BASED ON the dynamic headphone the frequency is going to be a bit off, as the ohms change based on frequency with the manufacturer trying to get it around the ohms it's stated. So different Dynamic headphones are going to sound different.

PLANAR HEADPHONES: Planar headphones, have a CONSTANT impedance, so what it says it what it gets. If you change the taps on a Planar headphone, you're likely not going to see any difference in frequency response, but you will be pushing the amp harder then it needs to if your choosing taps higher then the headphones impedance. Thus, faster fatigue on the tubes as you're pushing them harder if you choose to use higher taps then the headphones ohms. This is unfortunately what most people do because you don't have a preamp that pushes out 8 volts. YES 8 volts!

McIntosh modern preamps push out 8 volts, most people's equipment is 2 volts! If you have an RME ADI pro or higher make sure to use the +24 db in Total Mix or use a preamp with a high output preamp if you want the cleanest signal at the 12 o'clock position. If the RME UCX II or the ADI non pro use the +18, which will almost get you there. (Even with 8 volts you may not get the full 500mw at unity, they likely have the 500mw at the 1 o'clock position as this is the MAC sound.) I might test this when my new oscilloscope comes in and it motivates me to do so.

In other words and WHY I bought this amp, this amp gives you A LOT of variation to tune your sound. Once you like the sound you like, adjust your preamp AROUND that sound. If you want it to be more warm, then turn up the gain knob and adjust your preamp around that gain knob for volume control. What I do is I adjust the sound to taste with my preamp set a bit lower, so I have adjustability, then set the gain to where I like it, then raise the volume on the preamp to where I usually listen, then I don't change the gain knob. This give me some room on the preamp volume to adjust based on if I want more or less sound given my mood. When someone says this amp is to dark then likely they are using the gain knob at a higher amount. If they say it's to clean, then not enough gain.

Hope this helps some people and chime in if I got something wrong here.