Listener fatigue

Being bright is only one aspect of listener fatigue. If the treble control cannot tame it, there is probably an annoying upper midrange resonance present. This would be a peak in the frequency spectrum caused by poor isolation of the internal components such as the transformer or filter caps. Your original components are high quality and their construction takes this into account.

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.