Listener fatigue

This is a severly underated quality. Classe, Sonus Faber, and MIT give me that refined soft on the ears sound. I think brightness is the number one cause for listener fatigue.

I've tried Belles 150 both versions (including hotrod)when I was waiting for my new amp from dealer. I couldn't listen to even cozy music for more than one side of vinyl. Belles 150 has emphasized mid-highs and mid-base, doesn't realy have a dynamics as SS amp 150W/ch should have.

your amps' transformers can induce the parasite currents onto the components standing right above even on the different shelf especially on digital source. Try to find for your Heath amp other place rather placing it under preamp or source. If you have a possibility to turn your EQ off than it'll be extra help for internal element isolation.
Heath isn't the only exception. In that list you can add a bunch more even expancive ones.

So my first thing to do is to isolate power amp from any source or preamp component by at least 1/2'
The second would be to go with no EQ.

My experience agrees with what Blueswan points out. I owned a Simaudio Moon I-5 integrated for a year and also acquired an Electrocompaniet ECI-3 one month ago. I compared the two amps for three weeks.

The I-5 could at times be utterly brilliant, but sometimes exhibited an upper midrange glare and glassiness through my ProAc Tab 50 Sigs. While I was often enthralled with the I-5s qualities, I sometimes found my self cringing at the occasional brightness. Rock was almost unacceptable through the I-5which sounded lean and bright, which isn't a good companion to loud music. On the other hand, soft passages were remarkable with the Simaudio.

The ECI-3 on the other hand is never bright, and therefore condusive to long listening sessions without fatigue. I sold the I-5 and now live happily with the ECI-3. I sometimes miss the magic of the Simaudio, but the Electrocompaniet does indeed have magic of its own. Much of the air and soundstaging of the I-5, with a fuller and more natural midrange to boot. Plus the added benefit of no listener fatigue.

I think the difference was in part due to the slightly lean balance of the I-5 in comparison to the ECI-3. But I also don't discount the possibility of system synergy such as source, cables and isolation and not to mention "hot" room acoustics.

Early digital REALLY caused listener fatigue with me. Anyone remember Columbia's "Disc Quieting" process back in the 1980's?

Maybe I've been lucky, but I haven't had problems with fatiguing components, but I have gotten serious listeners fatigue from excessively bright ICs, speaker cables, digital cables, and worn out tubes in my SF pre-amp. I use and like Synergistic Research ICs and cables and use them throughout my system, but they have models such as Looking-glass ICs and Signature #3 cables that simply do not work in my system (tube pre-amp w/ SS amp), and definitely produced a brightness that caused significant listeners fatigue.

Some power cords will do it also, but I don't specifically remember the ones I didn't like. Auditioning wires in YOUR SYSTEM is essential to avoid this and other problems IMO.

In addition to excess brightness causing fatigue, an overly detailed, thin, analytical presentation will promote fatigue for me, but then these characteristics are also often related to excess upper mid-range and treble energy or brightness too. Cheers. Craig.