Any suggestions re; KiT6550 brightness?

[Intenso]

Hi all,

I wonder if anyone may have any suggestions for my KiT6550 amp that has developed a troubling brightness that is bothering me. The amp has all of the WAD recommended upgrades and also has Sowter Txs. The work has been completed by my friend Greg, so I can vouch for the quality of the work completed.

Since I’ve had the amp it has had a bit of an intermittent aspect to its sound quality. There have been times when it has sounded sublime with a very fulsome bloom to the bass and a sweet top end that I have very much enjoyed. At other times the bloom has disappeared, and the sound become toppy.

We recently changed the PSU caps to Mundorf Mylitcs and for about three weeks the sound was the best I have ever heard. The lovely bloom and sweetness were returned. However, since then the sound has again become toppy and lost the bloom. I am at a complete loss as to what the issue may be and wondered if this might be a burn in issue for the new caps. I am a little concerned as the amp has had at least a 100 hours use since the new PSU caps.

I’m wondering if the coupling capacitors may need replacing. They are currently Jensen PIO but are over 10 years old. However, they appear to be visually OK. Tubes are all relatively new.
I thought maybe the silver signal wiring. However, when the amp has been singing this hasn’t been an issue re; brightness.

Any ideas?